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Out-Q-sight! 




Shure's tiny new SM62 microphone does its own vanishing act in interviews 
and on stage. Less than five inches long, the SM62 slips out of sight behind 
podiums and set decorations. But don't let the small size fool you ... its com- 
bination of uncolored response and uniform cardioid pickup pattern pro- 
vides excellent performance characteristics and minimizes feedback. Field 
tested in difficult situations, such as rostrums at political conventions, the 
Shure SM62 has proved its versatility and dependability as "the little micro- 
phone with the big features." 



Shure Brothers Inc. 

222 Harlrey Ave., Evanston, IL 60204 

In Canada: A. C. Simmonds & Sons Limited 



I— ILJFR 



Manufacturers of high fidelity components, microphones, sound systems and related circuitry. 

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month 



db roams afield in April, consider- 
ing remote recordings of live per- 
formances. 

• Well-known lecturer Don Davis, 
with Ron Wickersham, discuss Ex- 
periments IN Enhancement, the 
delicate process by which correctly 
placed amplification equipment ena- 
bles artists, parlicLilarly those using 
synthesizer-computer instruments, to 
control creatively the enhancement of 
their musical interpretations. 

• R. A. NcilsOTi and Bobby Gold- 
stein report on an achievement by 
the Wally Heider people, recording 
live a combined Beach Boys and Chi- 
cago concert under terrific pressure 
of time and complication. The special 
ingredient of the professional, along 
with expertise, is pinpointed by the 
authors as Zen and the Art of 
Recorijing. 

• Shifting to the broadcast scene, 
Patrick S. Finnegan discusses Dolby 
B AND F.M, in his column. Add to 
this our other regular columnists, Nor- 
man Crowhurst. Martin Dickstein, 
and John Woram. 



• An unusual combination of cre- 
ative lighting and experimental pho- 
tography has produced this colorful 
montage of a Hammond organ key- 
board. (Credit: H. Armstrong Roberts) 



17 
24 
32 
37 



2 
4 
4 

6 
8 

13 

15 

21 
41 
44 




THE SOUND ENGINEERING MAGAZINE 
MARCH 1976, VOLUME 10, NUMBER 3 



F.M. STEREO SEPARATION 
Patrick S. Finnegan 

UNDERSTANDING HARMONIC DISTORTION 
Marc Saul 

FREQUENCY SHIFTERS FOR PROFESSIONALS 
Harald Bode 

BEING PRACTICAL ABOUT FEEDBACK, part 3 
Norman H. Crowhurst 



INDEX TO ADVERTISERS 

LETTERS 

CALENDAR 

FREE LITERATURE 

THEORY AND PRACTICE 
Norman H. Crowhurst 

THE SYNC TRACK 
John Woram 

SOUND WITH IMAGES 
Martin Dickstein 

NEW PRODUCTS AND SERVICES 
CLASSIFIED 

PEOPLE, PLACES, HAPPENINGS 



db is listed in Current Contents: Engineering and Technology 



Robert Bach 

PUBLISHER 

Bob Laurie 

ART DIRECTOR 

Eioise Beach 

CIRCULATION MANAGER 

Lydia Anderson 

ASST. CIRCULATION MANAGER 



Larry Zide 

EDITOR 

John Woram 

ASSOCIATE EDITOR 

Hazel Krantz 

COPY EDITOR 

Ann Russell 

PRODUCTION 



GRAPHICS Crescent Art Service 

db. the Sound Engineering Magazine is published monthly by Sagamore Publishing; Conipan>. Inc. Entire 
contents copyright t) 1976 by Sagamore Publishing Co.. Inc. 1120 Old Country Road, Plainvicw, L.I., N.Y. 
11803. Telephone (516) 433 6530. db is published [or those individuals and firms in professional audio- 
recording, broadcast, audio-visual, sound reinforcement, consultants, video recording, film sound, etc. Appli- 
cation should be made on the subscription form in the rear of each issue. Subscriptions are S7.00 per year 
(S14.00 per year outside U. S. Possessions, Canada, and Mexico) in U. S. funds. Single copies are $1.00 
each. Controlled Circulation postage paid at Harrisburg, Pa. 17105. Editorial, Publishing, and Sales Offices: 
1120 Old Country Road, Plainview. New York 1IS03. Postmaster: Form 3579 should be sent to above address. 



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Delta-T- 

A Dynamite 
Mixdown Tool 



CD 
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That's what we provide in our new Series 1 02 Digital Delay Systems. We've 
been mai<ing high quality, reliable delay systems for five years and have 
learned how to do it better than anybody else. 

Simply put, the Deita-T's 90 dB dynamic range and low distortion 
deliver a superb quality signal, leaving you free to creatively explore the 
powerful artistic potential of time delay. Discover for yourself, as leading 
studios such as Leon Russell's Shelter Studio have, how a Delta-T can 
thicken vocals and instruments, add slap or in-tempo percussive repeats, 
and provide ambience and spatial depth to the dry mono sources en- 
countered at mixdown. 

in the Delta-T 102 Series we have used our patented digital tech- 
niques to provide reliability, convenient features, and excellent performance 
at highly competitive prices. Let us help you define the configuration you 
need to get started. Call or write for more information. 



index qf 
aduerbs€ts 

Clear-Corn ....... 6 

Clover Systems 30 

Community Light & Sound . . 23 

Electro-Voice 8 

Garner Industries . . . 13, 20 

Gotham Audio 10 

Infonics 6 

Inovonics 22 

Jensen Tools 20 

J, B. Lansing 9 

Lexicon 2 

Micmix 27 

Neumann 10 

Orban/Parasound 39 

Peavey Electronics . . . Cover 3 

Precision Electronics .... 15 

Ramko Research . . . . 17, 19 

Rauland-Borg 18 

Recording Supply Co 12 

Revox 7 

Robins Industries 20 

Sennheiser Electronics .... 14 
Share Brothers .... Cover 2 

Sound Technology 29 

Standard Tape 4 

Stanton Magnetics 16 

Willi Studer 3, 31 

Tandberg 5 

Teac Cover 4 

Telex Communications ... 11 

Waters Mfg 12 

White Instruments 4 

Woram Audio J 8 



lexicon 

60 Turner Street 

Waltham, Massachusetts 02154 

(617) 831-6790 



sales offices 

THE SOUND ENGINEERING MAGAZINE 
New York 

1120 Old Country Rd. 
Plalnview, N.Y. 11803 516-433-6530 



Roy McDonald Associates, Inc. 
Dallas 

Stemmons Tower West, Suite 714 
Dallas, Texas 75207 214-637-2444 



Denver 

3540 South Poplar St. 
Denver, Colo. 80237 303-758-3325 



Houston 

3130 Southwest Freeway 
Houston, Tex. 77006 713-529-6711 



Los Angeles 

500 S. Virgil, Suite 360 
Los Angeles, Gal. 90020 213-381-6106 



Portland 

2035 S. W. 58th Ave. 
Portland, Ore. 97221 503-292-8521 



San Francisco 

Suite 265, 5801 Christie Ave. 
Emeryville, Cal. 94608 415-653-2122 



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the 

affordable 
Studer 



The new generation of professional STUDER 
tape recorders is designed for the use in broad- 
casting, television and recording studios as 
well as theatres and scientific laboratories. 
The low-cost STUDER A67 includes a wide 
range of modern features: 

3 servo controlled AC motors - Crystal con- 
trolled capstan servo - Vanable tape speed 
(2/4". . . 2 2 Vz") with external frequency -Tape 
tension control during all operating modes - 
Control logic with memory - Illuminated push 
buttons - Remote control of all tape transport 
operating modes - Automatics for continuous 
program - Mechanical counter, indicating Min 
& Sec - AC-Mains supply 50 or 60 Hz. 
110. . .250 Volts - Opto electronic end of tape 
sensor - Head block with aluminium die-cast 
frame - Tape lifter, may also be operated 
manually - Long life heads - Audio electronics 
module with plug-in cards in front of tape 



deck - Playback, record and bias amplifier 
boards have all necessary adjustments acces- 
sible from the front of the recorder - Switchable 
for equalization CCIR or NAB - Optional: VU- 
Meter/panel with peak indication (LED) - Head 
phone jacks - Available with or without VU- 
panel. as portable or console version or as 
chassis for 1 9" rack mounting - '/2-inch, 4 
track version in preparation 




WILLI STUDER AMERICA INC. 

Professional Audio Equipment. 

1819 Broadway. Nashville, Tennessee 37203. 

Phone 615-329-9576. Telex 55-4453. 

In Canada. STUDER REVOX Canada Ltd., 

phone 416-423-2831. Telex 06-23310. 



a 



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Our selling premise is 

Simple,,, 

STL magnetic Test Tapes 
are the Most Comprehensive 

We offer precision magnetic the \A/^Orlcl 

test tapes made on precision 

equipment for specific jobs in 1 " and 2" sizes as well as 

flutter tapes and all other formats. 

When you use STL test tapes you combine interchange- 
ability with compatibility. You know you are using what 
other leaders in the professional recording, equipment 
manufacturing, government and educational agencies 
throughout the world are using. 

Make sure your system is in step with the rest of the industry. 

Write for a free brochure and the dealer in your area. 

Distributed exclusively by Taber Manufacturing & Engineering Co. 



SUL 



ISTANDARDTAPE LABORATORY, Inc. 



2081 Edison Avenue 
San Leandro, CA 94577 
(415) 635-3805 



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NEW AUDIO SPECTRUM MONITORl 

'/a OCTWE REALTIME ANALYSIS MODEL 142 

■ PEAK READINQ • TWO MEMORIES: CUMULATIVE OR SAMPLE • 
VARIABLE TIME CONSTANTS (M ■ 3 SEC.^ * CALIBFWTED IN 
FEATURES D&M • lO^O-^Q DB DISPLAY RANGE 40 H2 to ia KHZ ON; lA 
OCTAVE ISO CENTERS * 11 x 2B LED ARRAY • BUILT^lM PINK 
MOISE SOURCE • 3Vii" >! DEEP RACK MOUNT. 

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Aletleis 



The Editor: 

I have at least six Scotch metal lOV^ 
inch reels that defy any attempts to 
de-warp them. They are valuable reels, 
but right now they scrape against the 
stainless steel panel on my Revox All. 
Would any of your readers have com- 
ments? Thanks. 

R. Dennis Alexander 
Radex Productions 
1 10 South Carlisle SL 
Greencastle, Pa. 17225 



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CALENDAR 



MARCH 

21-24 National Association of Broad- 
casters Convention. Chicago, 
Illinois. Contact: NAB, 1771 
N St., N.W., Washington, D.C, 
20036. (202) 293-3500. 

29-31 NOISEXPO '76, Hilton Hotel, 
New York City. Noise and vi- 
bration control. Contact: NOIS- 
EXPO, 27101 E. Oviatt Rd., 
Bay Village, Ohio 44140. 
(216) 835-0101. 

APRIL 

5-9 Acoustical Society of Amer- 
ica. Washington, D.C. 
22 Acoustical Conference. Hun- 
garian Society for Optics, 
Acoustics, and Cenematog- 
raphy. Budapest, Hungary. 
26-27 Acoustical Problems of Liglit- 
Structure Construction of Build- 
ings. Acoustical Commission 
of the Hungarian Academy of 
Sciences. Budapest, Hungary. 

MAY 

1 Midwest Acoustics Conference. 

One-day meeting covering sig- 
nal processing and data reduc- 
tion technology for solving tech- 
nical and legal problems in 
acoustics. Norris Center, North- 
western University, Evanston, 
III. Contact: H, O. Saunders, 
Rm. 24A, 225 W. Randolph 
St., Chicago, 111. 60606. (312) 
727-4331, 
4-7 Audio Engineering Society 
Convention, Hilton Hotel, Los 
Angeles, Ca. Contact: A.E.S., 
60 E. 42nd St., New York, 
N.Y. 10017. 
28-31 Sound and Vision '76. Bir- 
mingham, England. 



TANDBERG ^^^^BBS^^B 

10XD bridges the gap between consumer 
and professional tape recorders. 

Meet the world's first and only reel tape recorder that operates at 15 ips and combines 
Tandberg's unique Cross-Field recording technique with the world-famous Dolby* B system. 
Result: A guaranteed minimum signal-to-noise ratio of 72 dB, measured on a 4-track machine 
using I EC A-weighting. Simply put, the 10XD completely eliminates audible tape hiss! 

• Peak reading meters Remote control and rack mount 

• Direct transfer from playback to optional. Pitch control by special order. 



Here are some of the many sophisti- 
cated features that make the lOXD the 
finest tape recorderTandberg has ever 
built: 

• 3 speeds: 15, 7% 3% ips. Electron- 
ically selected 

• 3 motors; Hall-effect capstan motor 

• 3 heads; plus separate bias head 

• Electronic servo speed control 

• Electronic logic mode controls, 
including photo optics 

• Electronic balanced microphone 
inputs 

• Echo, sound-on-sound, editing, 
A and B tests 



record (flying start) 
' Ferrite playback head with symmetri- 
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ling purposes and differential 
playback amplifier 



fot a complete demonstration of this 
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INCLUDED IN PRICING 

Mid Atlantic Service Center 

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199 Davis Avenue 

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Phone: (301) 922-8865 




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759 Harrison Street, San Francisco Ca. 94107 
(415) 989-1130 



FREE LITERATURE 

GRAPHIC ART TAPES 

• Formaline plastic art tapes, de- 
scribed in this booklet, come in vari- 
ous colors and widths. Suitable for 
charts, graphs, printed circuit boards. 
Mfr: Graphic Products Corp. 

Circle No. 96 on R.S. Card. 

DIRECT DRIVE MOTORS 

• Beaii torque and hysteresis synchro- 
nous motors for use in tape recorders, 
audio turntables, video recording 
equipment, etc. are detailed in a 6- 
page brochure. Mfr: UMC Electron- 
ics Co. 

Circle No. 97 on R.S. Card. 
HEAT SINKS 

• Engineering drawings and thermal 
performance data on plastic power 
heat sinks are covered in a four-page 
brochure. Mfr: Thermalloy, Inc. 

Circle No. 98 on R.S. Card. 

BACKGROUND MUSIC 

• Description of a library service of- 
fering prerecorded background music 
is contained in this brochure. Mfr: 
MusiCues Corp. 

Circle No. SO on R.S. Card. 

DROP-IN MIXERS 

• A line of double balanced micro- 
strip and stripline tinj' drop-in mixers 
is covered in a two page product sheet, 
*DM-1005. Mfr: RHG Electronics 
Laboratory, Inc. 

Circle No. 81 on R.S. Card. 

OPTO-ISOLATORS 

• A 24-page short-form catalog lists 
complete specifications and applica- 
tions for opto-isolators and photoelec- 
tric control equipment. Mfr; Sigma In- 
struments, Inc. 

Circle No. 82 on R.S. Card. 

MEASUREMENT INSTRUMENTS 

• Myriad applications and full de- 
scriptions are included in this ambitious 
48-page catalog, TM 500, covering 
counters, digital multimeters, signal 
sources, power supplies, signal proc- 
essors, and oscilloscopes. Mfr: Tek- 
tronicx. 

Circle No. 83 on R .S. Card. 

NOISE ABSORBERS 

• Noise absorbers, barriers, and damp- 
ing materials are cataloged in this 8- 
page booklet. Mfr; Ferro Corp. 

Circle No. 84 on R.S. Card. 

STUDIO ACCESSORIES 

Preamps, equalizers, transformers, 
and microphone accessories are listed 
in a closely packed 36-page booklet. 
Mfr: Sescom. 

Circle 85 on R.S. Card. 



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entific analysis 
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Electronic music synthesis 
Noise analysis 
Film synchronization 
Radio telescopy 
Language laboratory 
Machine tool control 
Phonetic analysis 
Radio telemetry 
Industrial research 
Information retrieval 
Electrocardiography 
Making calibration tapes 
Tape mastering with SELFSYNC 
Data storage from digital computers 



And that's a simple statement of fact. 

From the moment it was introduced, 
the Revox A77 was hailed as a 
recording instrument of unique quality 
and outstanding performance. The 
magazines were unanimous in their 
praise. Stereo Review summed it all up 
by saying, "We have never seen a 
recorder that could match the perform- 
ance of the Revox A77 in all respects, 
and very few that even come close." 

So much for critical opinion. 

Of equal significance, is the fact that 
the Revox A77 rapidly found its way 
into many professional recording 
studios. 

But what really fascinates us, is that 
the A77 has been singled out to 



perform some unusual and highly 
prestigious jobs in government and 
industry. The kinds of jobs that require 
a high order of accuracy and extreme 
reliability. 

Take NATO (the North Atlantic 
Treaty Organization) for example. 
When they wanted a machine to stand- 
ardize on, a machine that would lend 
itself to use in a wide variety of circum- 
stances. And most importantly, a 
machine that was simple to use, the 
logical choice was the Revox A77. 

Or take the governmental agency 
that wanted an unfailingly reliable tape 
machine to register and record 
satellite bleeps. The choice? Revox. 

Or the medical centers that use 



specially adapted A77's for electro- 
cardiographic recording. 

We could go on and on (see accom- 
panying list), but by now you probably 
get the point. 

No other Va" tape machine combines 
the multi-functioned practicability, 
unfailing reliability, and outstanding 
performance of a Revox. 

If you have a special recording 
problem that involves the use of '^i" 
tape, write to us. We'll be happy to 
help you with it. 

And if all you want is the best and 
most versatile recorder for home use, 
we'll be glad to tell you more about ^ 
that too. 



R^OX 



0 



< 



Address 



Name. 



Revox Corporation in USA: 155 Michael Drive, Syosset, NY 11791 
For other countries: Revox International, 
Regensdorf 8105 ZH Althardstrasse 146, Switzerland 



State . 



Zip. 



*As and when available from our dealers 



Q. 



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IT 



(O 

-si 
05 



www.americanradiohistorv.com 



Number 98 in a series of discussions 
by Electro-Voice engineers. 




cuniNG 

THE . 



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05 



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I. 

CO 



CO 



AeiI. rridbsC liiiiir 



Under a project started several years ago at 
Electro-Voice, we have made extensive lab- 
oratory and field studies to determine the 
important performance characteristics desired 
in wireless microphone systems, problems to 
be overcome, and optimum operating pos- 
sibilities within the present state of the art or 
with improved materials and techniques. 
Reliability and flexibility of operation were 
the primary needs of most of the users we 
talked to. 

Applications are almost infinite, and the 
wireless system must work under the most 
adverse conditions. Broadcast quality audio 
must be provided over distances up to a third 
of a mile. The equipment must sometimes 
operate with ten other wireless systems on 
adjacent channels inside a theatre. The trans- 
mitter may be concealed in a chorus girl's 
costume in Vegas or placed in the back pocket 
of an actor going to the brink in a new 
disaster movie. As one of our contacts said, 
"When I shove the performer on the set, the 
equipment has to work the first time for the 
whole take without intermittents, without 
fadeouts, and without being knocked out." 

Reliability has been increased in the new E-V 
wireless microphone equipment by careful 
attention to details and use of the best 
available materials. Lemo Quick-Lok con- 
nectors on the mike and antenna leads pro- 
vide a superior flex and strain relief over other 
types of connectors in use. The transmitter 
itself is small, rugged, and carefully shock 
insulated inside a diagonally drawn sectional 
aluminum case. It will withstand being sat on, 
even being dropped, and continue to work. 

A significant increase in radiated output over 
other wireless mike systems reduces r.f. inter- 
ference problems, enabling clean, non-fading 
reception over more than normal distances. 
In addition, radiated output power can be 
doubled to 100 mw via a simple switching 
arrangement in the transmitter to take care 
of really severe conditions or range. By 
observing a built-in LED indicator, mike 
gain control is adjustable to allow optimiza- 
tion of signal-to-noise ratio and dynamic 
range. A double-tuned helical resonator RF 
preselector in the receiver as well as an 
"AUTOLOCK" discriminator prevent drift 
and out-of-band interference. Any E-V 
dynamic or electret condenser microphone 
may be used with the Model 221 transmitter, 
for great versatility and best performance. 
The unit even provides a bias voltage for 
electrets, eliminating any separate micro- 
phone battery. 

ElecfroT/bice:. 



company 



Dept. 363BD,603 Cecil Street 
Buchanan, Michigan 49IU7 

Circle 19 on Reader Service Card 



Alheoiy&piaclice 

V m NORMAN KCROWHURSr 



AMPLIFIER GAIN A 



FEEDBACK FRACTION 8 



Figure 1, The classic teeiiback block schematic- 
voltage In, voltage out. 



• Recently, I received a letter ques- 
tioning my use of the word "medi- 
ate" and asking about Nyquist dia- 
grams, referring to my article on 
Feedback in the November issue. 
The better known meaning for "me- 
diate" is to act as an intermediary. 
We grew up with that meaning. But 
educators are always coining words. 
They use the word mediate to mean 
"put into media form." 

Thus, when a lesson presently 
printed in a book is dictated onto 
tape, for example, it is being "medi- 
ated," in educational parlance. 

Regarding the Nyquist diagrams, 
the reader admits that he should look 
back at the Part 1 of the series, be- 
cause he only has difficulty with part 
2. It is so difficult to keep from 
bringing up what I've said before. In 
Part 2, I started from the formula 
developed in Part 1 (September is- 
sue). And back last year, I com- 
mented on the little interchange about 
the use of formulas that occurred 
during the summer session at Brig- 
ham Young University. 

What those students wanted was 
all the relevant formulas, so they 
could "plug them in" in the appro- 
priate places for the audio systems 
on which they worked. My response, 
in brief, was to indicate that they 
need, far more, to understand what 
they are doing. Now this reader's 
query about Nyquist just confirms 
what I was saying there, once more. 

In part 1 of the series, I gave sche- 
matic diagrams of feedback ampli- 
fiers, using series and shunt deriva- 
tion of the feedback signal, at the out- 
put, and also series and shunt injec- 
tion of the feedback signal, at the 
input. From this, in each instance, I 
derived the fomula that showed the 
effect of feedback on amplifier gain. 

Also, in Part 1, I deliberately stayed 
away from phase angles, assuming 
just for simplicity, that feedback is 
always either positive or negative, 
never "in between." Of course, the 
facts of life are that there is never a 
feedback system that does not have 



PHASE WITH FEEDBACK 




Figure 6. The construction for the Nyquist diagram for 
criterion of stability, 

in-between conditions as well. And 
that is what Nyquist diagrams are all 
about. But I kept that for Part 2. 

As I had discussed the formula 
pretty well in Part 1, I felt that Part 
2 could assume that Part 1 had been 
read and apply the formula to cases 
where feedback is not just positive or 
negative, but where it goes in be- 
tween. In the series, I used more or 
less conventional symbols, mainly for 
the benefit of people who may have 
learned the subject before, but never 
understood it. But I am well aware 
of the difficulty of relying on for- 
mulas to convey a picture of what is 
happening. Vectors run into a similar 
problem, mainly because of the poor 
way they are too often taught. 

Let us take a look at Figure I, 
reproduced from the September is- 
sue. If you don't like all those sym- 
bols, disregard everything except the 
input end, for the moment, where 
you have three voltages: that at the 
input to the amplifier, labeled e^; that 
coming from the feedback network, 
labeled /3e„; and that across the com- 
bination, which is what must be ap- 
plied as input to the whole system, 
labeled e^. 

The equation following e,j merely 
says, in algebraic terms, that these 
three voltages must jibe. Thus, if the 
internal input to the amplifier is I 
mV, the fed back signal is 9 mV, and 
the feedback is negative, the external 
input to the amplifier, e^, must be 
9 -I- 1 = 10 mV. There is no way it 
could come to something different. 

That could apply to d.c, or to a.c. 
of some frequency, which in general 
it more often does, producing what 
we call signal. We can think of d.c. 
and of signals of different frequen- 
cies, one at a time, but that simple 
formula must always be true. The 
voltages must add up round the 3- 
sided loop, at the input to the am- 
plifier. 

POSITIVE FEEDBACK 

That was for the case of negative 
feedback. What about positive feed- 



www. americanradiohistorv.com 




Introducing The Ice Cube. 



It can go all day and all night and 
still keep its cool. Here's why: 

One, there's a super quiet, 
thermally activated two-speed fan that 
runs low most of the time, but kicks 
into high when the going gets 
hot. (And, at a short 5Va" tall. The Ice 
Cube is perfect for stacking.) 

Two, there's an absolutely 
exclusive 2000-watt solid-state 
inverter power supply instead of those 
massive transformers you're used to. 
Total weight: 35 pounds! 



There's more. 300 watts RMS per 
chann e l, both chann els driven into four 
o hms from 20Hz to 20 KH z, at .05% or 
less total harmonic d istortion. 

Color-coded peak reading lights 
step up and down so you're the first to 
know if it's clipping. 

Go see The Ice Cube. It's formal 
name is the JBL 6233 Professional 
Power Amplifier. Bring $1500 and it's 
yours. 



UBL 



Circle 20 on Redder Service Card 
www.americanradiohistorv.com 



theory & practice (cont.) 



back? Well, that can cause oscilla- 
tion, which is why we need, later on, 
to get into Nyquist plots. But first 
take the simple instance, where we 
know feedback is positive, instead of 
negative. If the amplifier input is still 
1 mV, internally, and the feedback is 
also 1 mV, but positive instead of 
negative, then we do not need any 
external volts at all for the amplifier 
to oscillate. If is zero, the feedback 
will provide the input voltage di- 



rectly, and signal will go on forever 
— oscillating. 

If feedback is positive, but less 
than equal to the original input, gain 
is increased instead of reduced. Thus, 
if now the internal input voltage is 
10 mV, and the fed back signal is 5 
mV, positive feedback, then the ex- 
ternal input voltage needs only to 
provide the other 5 mV to make up 
the 10 total. Gain has doubled, be- 
cause it takes 5 mV to produce the 
same output that 10 mV did without 
feedback. 

If feedback, using the same exam- 



QUKK. 

HOW MUCH 
DOES A NEUMANN 
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pie, is 8 mV, then gain is 5 times, 
because now it takes only 2 mV to 
produce the same output. If feedback 
is 2 mV, then gain is increased by 
25 per cent, because we get the same 
output for 8 mV input instead of 
10 mV. 

PHASE 

We are still with Part 1, conveni- 
ently ignoring phase. But phase won't 
go away because we choose to ignore 
it. As Part 2 started out explaining, 
whenever you use a capacitor or in- 
ductor, or something that has those 
properties, there are always phase 
shifts waiting to come out at some 
frequency or other. 

There is no way to get a roll-off 
without phase shift, although there 
are ways to get phase shifts without 
roll-ofTs, which is another whole story. 
If you will now look at Figure 6, 
which was in Part 2, you can see how 
the same idea, easily accommodated 
by simple addition or subtraction 
when feedback is conveniently either 
pure positive or pure negative, can 
be applied to other phase combina- 
tions. 

Now, the internal input voltage is 
that shortest line between 0 and —1. 
The feedback voltage is the line la- 
beled Ap. And the third side of that 
triangle, labeled 1 + A/3, is the ex- 
ternal input voltage. These three must 
jibe by forming a closed triangle, be- 
cause we have those three points in 
the circuit. 

Figure 6 assumes that all the 
phase shift is in the amplifier, none in 
the feedback. So as well as being the 
fed back signal, the line A/3 will have 
the same phase as the output voltage. 
Without feedback, the input voltage 
is the line between 0 and —1. But 
with feedback, it becomes the line 
labeled 1 -I- A/3. So this diagram 
enables us to show the effect of feed- 
back on phase as well. 

When we considered the simple 
positive or negative feedback cases, 
we showed that if the feedback signal 
is equal to, or greater than, the input 
signal, and positive in phase, the am- 
plifier will oscillate without benefit 
of any external signal. In terms of 
the diagram at Figure 6, this means 
that the line labeled A/3 will swing 
round to right until it passes through 
the -1 point to extending beyond it. 

The Nyquist plot is made by con- 
structing many of these diagrams, 
one for every possible frequency, and 
then joining up all the points where 
the apex of the triangle comes. The 
curve so formed is what mathemati- 
cians call the locus of the point, 
which merely means it is a curve 
showing how the point moves, as fre- 



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theory & practice (cont.) 

quency, which is the independent 
variable, is changed. 

Now a question some ask is, "Will 
the amplifier oscillate, if the fre- 
quency at which oscillation occurs is 
not present?" The answer is yes. 
Why? You've heard of noise, un- 
undoubtedly. No electronic device is 
without it. If well-designed, noise is 
very low, hopefully inaudible, but 
still there. And noise contains a ran- 
dom sampling of all frequencies. 

So, if at some frequency the line 
extends through the - 1 point, 
which means the curve its locus 
makes will encircle the — 1 point, 
then at that frequency the random 
piece of noise will "go around again" 
amplified to a bigger level. Each time 
around will make the signal bigger 
at that frequency, until the amplifier 
is in full fledged oscillation, limited 
by distortion that puts components 
of other frequencies into the signal. 

I hope that this additional expla- 
nation will help any readers who 
may, like the one who wrote in, have 
had difficulty with Nyquist. In a 
teaching situation it would be much 
easier. If instructional material is 
(what educators call) mediated, it 



can be made easier than it is, the way 
we are now doing it. 

When I wrote that three-part se- 
ries, although I tried to make it easy 
to follow, I was limited to using con- 
ventional written communication. 
The reader who wrote in asked a 
question that could have been raised 
immediately, had we been in class. 
Now — several months later — I re- 
spond to that question. Have I now 
made it clear? It will be months again 
before we know that. 

That is an advantage of using 
media in education, when it is prop- 
erly used. Responses can be built 
into the system. This kind of diffi- 
culty can be anticipated, and some- 
thing put in to start the student on 
finding his way out of it. The point I 
had been trying to make is that when 
material is intelligently "mediated" 
this can be done. 

In fact, since books are cheaper 
than mediated materials I see no ad- 
vantage in mediating, unless the medi- 
ated material does something the 
books could not do. Merely dictating 
books onto tape is not, in my opin- 
ion, "mediating." For this reason, 
audio people have a responsibility to 
do something about education. At 
least, that is the way I see it. ■ 



MOVING? 

Keep db coming 
without interruption! 

Send in your 

new address promptly. 

Enclose your old 
db mailing label, too. 

Write to: 

Eloise Beach, Circ. Mgr. 
db Magazine 
1120 Old Country Rd. 
Plainview, N.Y. 11803 



CORRECTION 
In Martin Dickstein's January col- 
umn, p. 18, line 6, "6328 Angstroms" 
was incorrectly referred to as "6,328 
degrees." The correct terminology is 
A. Angstroms are linear measure- 
ments, and canot be expressed as 
degrees. 




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the sync tK|M^ 



• Not too long ago, the New York 
section of the Audio Engineering So- 
ciety had a meeting on the advance 
of technology. The panelists held 
forth on the state of the art today, 
as compared to the earliest days of 
commercial stereo. 

The meeting came about as a re- 
sult of some frequently heard com- 
plaints about the sorry state of some 
of today's recordings. Of course, 
there were a lot of wretched record- 
ings released in the early days — most 
have long since achieved the oblivion 
they deserved. The good ones linger 
on though, and people sometimes ask 
why, after almost a quarter of a cen- 
tury of progress, these "golden old- 
ies" still stand up so well, especially 
in comparison to some very recent 
releases. Or, if some 1950's records 
are so great, why aren't more of the 
hits of the 70's at least comparable, 
if not better, in overall recorded 
quality? 

With the technology available to- 
day, we have the capability of pro- 
ducing great sound. But we also have 
the capability of thoroughly botching 
up a record. To prevent this, the 
mixer must be even more of a tech- 
nologist than before, and yet he can- 
not forget musical values either. How- 
ever, many of today's records sub- 
vert the music to the technology, as 
panelist Bert Whyte pointed out at 
the meeting. He happened to be talk- 
ing about some recent classical quad- 
riphonic recordings, but the remark 
applies as well to the top 100 scene. 

Often, the technology gets misused 
because the engineer doe.sn't have the 
background that his craft really 
should demand. Or, his producer is, 
to put it bluntly, an incompetent. 

At the meeting, someone referred 
lo the wealth of information avail- 
able in any well equipped technical 
library. Someone else pointed out 
that much of that information is 
written in "Tcchnicale.se" and can 
only be comprehended by people 
with advanced degrees, or a talent 
for the obscure. Faced with this 
mountain of difficult reading, the be- 
ginner is apt to throw up his hands 
in despair at ever finding a paper he 
can understand. 

Two things are needed. The first is 
to dispel the notion that in order to 
be a successful engineer, all you need 
to know is how to snap your fingers 
on the beat, and when to say "outa- 
sight" or whatever they say in your 



town. The second i.> some sort of 
guidebook through the academic jun- 
gle for those who really want to learn 
a little something about recording. 

Which brings us more or less to 
the point of this little epic. In work- 
ing on my book (subliminal plug) 
I've managed to accumulate a small 
collection of papers on this and that 
subject, one of which is plain old 
stereo. Some are more readable than 
others, and most have at least a little 
something of interest to the working 
recording engineer. Stereo may be 
old, but it's not so plain, and you 
don't really get it from a bunch of 
pan pots, contrary lo popular belief. 
Ft seems there's a lot more going on 
out there in papersville than many 
mixers dream of, and some of it may 
even help you get a little more out 
of your recordings. 

GUIDED TOUR THROUGH 
THE JUNGLE 

So-o-o, this is sort of a guided tour 
through at least a few of the papers 
that may be of particular interest. 
Authors have been writing on the 
subject for years, and much of it is 
relevant today, especially with multi- 
track technology. We can start off 
with a little quiz. 

1. How many cars do you have? 

a. 1 

b. 2 

c. 3 or more 

The correct answer is b. If you 
answered a or c, you're a special case, 
and this article is not for you. 

2. If you have only two ears, 
what are you doing with all that 
multi-track recording gear? (Essay 
type answer on this one) 

"I'm using it to create all sorts of 
beautiful music which would other- 
wise be impossible." 

3. When you get all finished cre- 
ating all sorts of beautiful music 
which would otherwise be impossible, 
how many ears will you have? 

3. a. 1 

b. 2 

c. 3 or more 

The answer to this one is also b. 

EARS 

So, no matter what you do or how 
you do it, it all comes back to two 
ears. Last month's db article by Dan 
Queen had a little something to say 
about how the ear works. Not just 
his ear, but yours too. 



For instance, let's say we're doing 
a mixing session, and want the guitar 
on track 15 to be right-of-center. 
The pan pot should take care of that 
nicely. But before you reach for it, 
think about what you would hear if 
the guitar was not on the tape, but 
in the room with you, sitting just to 
the right of your center line. 

Common sense tells you that you 
would hear the music with both ears, 
and the intensity difference from one 
ear to the other would probably be 
unmeasurablc. Yet you would know 
exactly where the guitar was located. 
Why? 

More than twenty years ago, Wil- 
liam B. Snow wrote: A complex wave 
pulse has an imtiul wavefioni which 
arrives at the near ear a short time 
before it arrives at the jar ear. It is 
this small time clifferetice which is 
used hy the hearing sense to deter- 
mine small angular variations, par- 
ticularly for sounds near the median 
plane (straight ahead) . . . The loud- 
ness differences at .mch small angles 
are negligible and it must be a.ssu/ned 
that the arrival-time differences give 
the localization elites.^ 

Note that Snow emphasizes the 
importance of time of arrival, rather 
than intensity. 

Now then, back to the pan pots. 



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the sync track (cont.) 

You've placed the guitar right-of- 
center by feeding track 15 to both 
speakers, but in unequal proportion, 
The sounds from the speakers arrive 
at your listening position at precisely 
the same time, and of course both 
ears hear both speakers, Each speaker 
tries to convince you that the sound 
is coming from it alone. As long as 
you remain well centered, your hear- 
ing mechanism doesn't have much 
trouble refereeing this psycho-acous- 
tical tug of war, and the localization 
is reasonably effective 

But if you move around, the gui- 
tar moves with you. In fact, if you 
move to that right-of-center location, 
the sound from the left speaker is 
now delaysd slightly — just the oppo- 
.site of what would happen if the gui- 
tarist was actually in the room, and 
you moved to a seat directly in front 
of him And, as you move closer to 
one speaker, it (apparently) gets 
louder. In a review pf the Haas Ef- 
fect, Mark Gardner describes what 
happens: // now, orte of the real 
sources is slowly moved farther away, 
the apparent source moves towards 
the other (nearer or earlier) signal. If 
one signal is made stronger than the 
other, a .similar movement will occur 
toward the louder signal, or an inter- 
change between level and time of ar- 
rival can be made within certain 
limits.^ 

Bringing all this back to the world 
of recording, it seems as though the 
pan pot is not the greatest direction- 
al tool in the world. As Gardner im- 
plies, it has "certain limits." But, 
when mixing down a multi-track 
tape, it may be all you've got at hand, 
even though in 1958, Fr. Heegaard 
wrote: It has generally been consid- 
ered that, in stereophonies, it is pre- 
ferable to rely on a single pair of 
microphones in order not to spoil the 
directional effect.^ 

Needless to say, this policy would 
severely cramp the style of a lot of 
contemporary recordings, but it's in- 
teresting to note that long before the 
birth of multi-track recording, some 
of its limitations were anticipated, 
at least in the literature. 

So where's the happy ending to 
all this? Maybe the literature also 
suggests a way to make better re- 
cordings, as well as telling us what's 
wrong with the ones we're making. 



TWO-MICROPHONE PICKUP 

Well, almost. There are many ref- 
erences to the excellent sense of 
stereo perspective when one or 
another type of two microphone pick- 



up is used. Carl Ceoen compared six 
different microphone placements (five 
stereo and one pan pot) and in most 
of his tests, the pan pot method was 
outranked by one or more of the 
stereo placements,^ Earlier, an ap- 
plication note from Gotham Audio 
Corp." described a method of mixing 
additional several stereo pairs to- 
gether. The technique is quite inter- 
esting, but needs a separate article to 
describe it fully. In practical terms, 
it has the disadvantage of requiring 
two tracks for each additional stereo 
mic if the engineer is not prepared 
to mix them together during the re- 
cording session. 

Since this practice is an unlikely 
one (especially on Sel-Sync sessions!), 
it may not be of much help to the 
modern we'll-fix-it-in-the-mix techni- 
cian who is trying to come up with a 
better recording. But, what about 
when it comes time to add the solo- 
ist? Maybe a little extra effort would 
pay off here. Perhaps some of the 
stereo techniques that have gotten 
pushed aside should be dusted off 
and tried. 

Can you spare two tracks for the 
soloist? Why not have him/her/it sing 
into a crossed pair of Figure-8s? If 
you've really got nerves of steel, have 
the chorus stand on the other side of 
the microphone and do their thing at 
the same time. If you can get the 
producer to listen before he has his 
coronary, he may actually like what 
he hears. Then you'll be ready for 
the real hard-core stuff, like miking a 
whole darn string section in stereo! 
Of course, the burden of musician- 
ship is then passed back to the musi- 
cians, who may not be ready for such 
a shock. But if you explain it very 
carefully, they may actually get en- 
thusiastic about playing real music 
again. Or they may walk out. It's 
happened before. ■ 



RKFERENCES 

1. Snow, William B. "Basic Principles 
of Stereophonic Sound," Journal of the 
SMPTE. vol. 61, November, 1953, p. 
567. 

2. Gardner, Mark B. "Historical Back- 
ground of the Haas and/or Precedence 
Effect." Journal of the Acoustical So- 
ciety of America, vol. 43, no. 6, p. 1243. 

3. Fr, Heegaard. "The Reproduction of 
Sound in Auditory Perspective and a 
Compatible System of Stereophony." 
E.B.U. Review, no. 52, 1958. 

4. Ceoen, Carl. "Comparative Stereo- 
phonic Listening Tests." Audio Engi- 
neering Society preprint no. 809, Oc- 
tober, 1971. 

5. Temmer, Stephen F. Applications 
Note. Gotham Audio Corp. 



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(fU ^sound with imiges 



• When wc began this discussion 
last month, I mentioned some of the 
complex installations of audio-visual 
systems designed by Hubert Wilke 
Associates of New York City. Most 
of them include various projectors, 
such as the overhead and the film 
units, and also one or more slide 
projectors. Some are front-screen, 
others have rear projection. Usually 
large facilities also include remote 
controls so that the presenter can 
advance or reverse the slides, start 
the film, play a tape, etc. Some con- 
trol units allow volume adjustment, 
light dimming, curtain movement, 
and random access among other en- 
vironmental and program operations. 

In addition to the projector, there 
are two other considerations that 
are also vital to a successful show- 
ing. One is the presenter himself; 
suggestions were made in the previ- 
ous column about ways someone in- 
volved with selling equipment or de- 
signing a, V facilities might be of as- 
sistance to the client by ofl:ering tips 
to help the presenter make a better 
showing. The other factor is the soft- 
ware to be used. No matter how so- 
phisticated the installation, and how 
polished the presenter, if the soft- 
ware falls flat, part of the message is 
bound to be lost, along with the ex- 
pense of the material itself. 

The production of films, or film- 
strips, is an art in itself. Many com- 
panies have been formed for the pur- 
pose of producing films for specific 
applications. Some specialize in train- 
ing material. Others produce travel- 
ogues, cartoons, commercials, or 
stock material for cutting into other 
films. These films can become quite 
expensive, depending on location, 
staff required, easting, length, edit- 
ing and laboratory work, and so on. 
There is one item, however, that can 
be deadly boring if no originality is 
exercised in the production . . . 
slides. 

SLIDES 

Almost all presentations made with 
a projection system include slides. 
The usual routine is to show a slide 
with words on it. Most presenters 
like to read the words, then talk 
about the subject. Others .show the 
slide but do not read it. They talk 
about the subject but leave the viewer 
to read for himself. This can prove 
to be very distracting for most of the 
audience since they don't know 



whether to read or listen, and thej 
can't do both. 

When there is only one projector 
in use for a single-image presenta- 
tion, there is the usual IV2 second 
black space between slides. If a mon- 
otonous presentation, one slide after 
another, is followed in a regular se- 
quence, it can become sleep provok- 
ing. For someone in the audience 
who just finished a heavy lunch with 
two or three drinks, it's like driving 
at night with heavy eyes and becom- 
ing mesmerized by the white dashes 
of the lane-dividing line flashing by. 
To prevent this, not only should the 
presenter be more animated to keep 
the audience's attention alert, but the 
equipment can be used to greater 
advantage, varying the length of time 
used for each slide to create greater 
interest and increased retentivity. 

When two projectors are available 
for side-by-side showings, this can 
add to the impact of the presented 
material. They need not be advanced 
simultaneously. Provision can be 
made to work one unit while the 



other remains stationary. This, then, 
allows a change of slide on the left 
while the right side is black. A com- 
plimentary slide can then come up 
on the right, and change several 
times while the one on the left is 
stationary. Then both can go, and the 
right side come up alone. 

It might also be an interesting ar- 
rangement, if a film is used with the 
slides, to have it displayed on one 
side, in place of one of the slides, 
instead of in the center. This way, the 
slide on the other side of the screen 
can mention the point under consider- 
ation while the film is playing. This 
leads to one more interesting possi- 
bility. The slide shown just before a 
film is to go on can actually be the 
first frame of the movie. This way, 
the film can overlap the slide for an 
instant before the slide is advanced 
to a black. It will look as though the 
slide had started to move from a still- 
frame of the film. 

THREE PROJECTORS 

Where a third slide projector is 
available, a variation that is possible 
is to have either three side-by-side 
slides, or, maybe even more effec- 
tively, an interlace of center screen 
images with two side-by-side slides, 
so the viewers' eyes have to vary 
their positioning toward the screen at 



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Top Disc Cutting 
Studios, lilce The 
Mastering Lah^ rely 
on Stanton's 681- 
Caiibration Standard 
in tlieir Operations. 



sound with images (cont.) 



different times. Sure, you can now 
add a second film projector for either 
a showing in the center or on the 
other side of the first film unit, but 
there is a limit. There is such a thing 
as overkill. If the effect to be pre- 
sented is for mood, or to indicate 
complexity of a situation, then any- 
thing may go. If, however, a definite 
message is to be presented, with facts 
to be remembered, let it not go over- 
board. But by all means, keep the 
audience awake by using the equip- 
ment or installation to its best ad- 
vantage. 

HORIZONTAL OF 
VERTICAL SLIDES 

Now that the equipment and the 
presenter are ready for the presenta- 
tion, how about the slides themselves. 
In the simplest setup, the single image 
from one projector, the slide format 
is 2:3 (height to width). The hori- 



Not everyone who plays records needs 
the Stanton Calibration Standard cartridge, 
but everyone who ma^fes records does! 

At The Mastering Lab, one of the world's 
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A recording lathe operator needs the 
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constant comparing of lacquer discs to 
their original source enables him to ob- 
jectively select the most faithful cartridge. 
No amount of laboratory testing can reveal 
true musical accuracy. This accuracy is 
why the Stanton 681 Series is the choice 
of leading studios. 

When Mike Reese, principal disc cutter 
at The Mastering Lab, plays back test cuts, 
he is checking the calibration of the cutting 
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and the lacquer disc. The most stringent 
test of all, the evaluation of direct to disc 
recordings, requires an absolutely reliable 
playback cartridge ... the 681 Triple-E. 

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srraNTon 



All Stanton cartridges are designed 
for use witli all two and four-ctiannel 
matrix derived compatible systems. 

Circle 28 on Reader Service Card 



zontal format has several advantages. 
When you consider the usual room 
with a flat floor and a 7-to 8-foot 
ceiling, and a seated audience, the 
bottom of the image should not be 
lower than 4 feet off the floor. Pos- 
sibly 3'/i feet, but if it is lower than 
that, the people toward the back will 
have great difficulty seeing the lower 
part of the picture. 

This allows a 3- to 4-foot high 
image. The width, correspondingly, 
would be AVz to 6 feet. This would 
permit good visibility to a last row 
somewhere about 27 to 36 feet from 
the screen, with application of the 
rule-of-thumb, 6x image width. With 
proper letter size (another story for 
sometime in the near future), and 
good artwork, there should be no 
problem getting an effective message 
displayed on the screen. In order to 
keep the slides moving with interest, 
it's best to use good pictorial repre- 
sentations such as photographs, in- 
stead of words where possible, logos 
instead of names, shapes in place of 
straight-forward typed copy on the 
slide, and so on. 

Since a good deal of the material 
usually read is vertical in shape, such 
as newspapers or magazines or books, 
or even advertisements, it sometimes 
is well to use vertical slides. However, 
in many presentation rooms, vertical 
slides would spread over the top and 
bottom of the horizontal-shaped 
screen and look bad. A vertical effect 
is possible, however, by shooting the 
vertical material in a horizontal for- 
mat but on a black background. This 
avoids showing the shape of the slide 
not being used. The copy has . the 



appearance of being vertical, and is a 
definite change from the other slides 
which may be horizontal. 

If you use white words on a dark 
background, a harsh contrast is cre- 
ated, causing eye fatigue after a while. 
In some cases, where only a few 
words or a symbol or logo are used, 
they might be on black for impact 
and change from the slides around 
it. But, especially, where the system 
is front-projection, harsh contrast 
just doesn't work well over a pro- 
longed presentation since the lights in 
the room are probably subdued, or 
close to dark. Even in rear-screen 
systems, continued viewing of sharp 
contrast, in spite of the fact that the 
lights can be left on in the room dur- 
ing the presentation, can be tiring. 
However, for effect, impact, varia- 
tion, and movement, contrast can be 
of some value. 

DISSOLVE SYSTEM 

A simple variation on the one- 
image theme is a dissolve system. 
This eliminates the IVi second black 
pause between views while the mech- 
anism advances the slides. A smooth 
movement between slides can be ef- 
fective in building a graph, for in- 
stance, or a chart, or a pictorial im- 
age. Starting with a single bar on a 
chart, dissolving into a two-bar chart, 
then three bars, etc., can be effective 
in showing growth. Dissolves from a 
small image to a larger one introduce 
movement and indicate growth. (In 
the case of the recent economy, per- 
haps the dissolves were shown in 
descending or receding order.) 

One way to show detail on a com- 
plex chart is to dissolve into close- 
ups of the desired section of the or- 
iginal large-scale image. All sorts of 
variations can be designed with great 
effect and a show of creativity. Of 
course, in a dissolve presentation, re- 
member that slides alternate from 
one projector to the other. This may 
cause some problems with changes in 
the presentation, especially if the 
changes include moving slides around 
and go up to the last minute, too. The 
presentation, however, will gain from 
variations in the material. 

You, as the supplier, installer, de- 
signer, recording engineer, photo- 
grapher, producer, user, or techni- 
cian, can really be of great help to 
the client if you can show him how 
equipment and systems, slides and 
software, and the presenter himself 
can help to improve the presentation 
and make the message get across . . . 
and stick. It takes all three to tango. 



www.americanradiohistorv.com 



PATRICK S. FINNEGAN 



F. M. Stereo Separation 



STAGE OR STUDIO 



RIGHT 

^ Dz^ 

MICROPHONES 
LEFT 



RIGHT AUDIO SYSTEM 



LEFT AUDIO SYSTEM 



HOME OR OTHER 
"REMOTE" LOCATION 



RIGHT 



SPEAKERS 



LEFT 



Figure 1. The basic purpose of stereo — a pair of "remote" ears. 



AVERY iMi'ORTANT element of a 
stereo system is separation of 
left and right audio channels. With- 
out separation, the two channels 
would blend into one and the system 
cease to be stereo. Maintaining sepa- 
ration is difficult enough in an ordi- 
nary audio system, but it is far easier 
than maintaining separation through 
an f.ni. transmitting system. 

SEPARATION 

An individual listening to a live per- 
formance on stage hears sounds from 
many directions. Since he normally 
hears with two ears, he can discern a 
sense of direction from which the 
sounds come. The stereo system at- 
tempts to capture these sounds on at 
least two microphones and direct the 
output of each microphone through 
separate channels into storage on audio 
tape or record, later reproducing chan- 
nels through two separate speakers. In 
other words, the system tries to pro- 
vide "remote" ears for the listener. 



Patrick S. Finnegan has had a long 
and distinguished career in broadcast 
sound. Beginning next month, 
Mr. Finnegan will contribute a 
regular column Broadcast Sound. 



These two channels, then, must faith- 
fully convey the original sounds as ob- 
tained by the microphones to a speaker 
output for each channel. If sounds are 
permitted to blend together haphaz- 
ardly, any place along the route, the 
channel separation will be lost. 

DETERIORATION 

Many elements occur between the 
two microphones and the final two 
speakers. Each element has its own 
limiting factors which can deteriorate 
or destroy the channels' integrity. Be- 
sides the microphones, there are the 
various amplifiers, the recording tape 
machines or disc cutters, the repro- 
ducing machines and amplifiers. When 
it is desired to send the stereo through 
an f.m. system, a great many more 
elements and limiting factors arc in- 
troduced into the chain. 

An f.m. system does not transmit left 
and right audio channels separately, 
as is done through an audio system. 
Instead, the left and right audio chan- 
nels are carefully blended together in 
a matrix system. After further proc- 
essing, the sound comes out of the 
stereo generator as a composite signal. 
It is this composite signal which ac- 
tually modulates the transmitter. Al- 
though the basic requirement of the 
stereo system is that the channels re- 
main separate, the matrix deliberately 
blends these two channels together. 




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WRITE FOR TECHNICAL BULLETINS 

RAULAND-BORG CORPORATION 

3535 W. Addison St.. Dept. N.. Chicago, III. 60618 

Circle 30 on Reader Service Card 




MATRIX 



PILOT 



SUPPRESSED 
SUB-CARRIER 



LEFT AUDIO 


L 


MODULATION 






(L + R) ■" 


INPUT 




OUTPUT 




R 


(L-R) 


RIGHT AUDIO 




TO 



Figure 2. The matrix blends the two 
audio c/janne/s fo produce SUM and 
DIFFERENCE signals. 



Herein lies its greatest potential for 
loss of channel integrity, for, if this is 
not done carefully, the channels can- 
not be recovered anil restored to their 
full integrity in the receiver. 

THE MATRIX SYSTEM 

Any broadcast system that is well 
established in public use is required 
by the FCC to provide a compatible 
signal when any additional service or 
modification is done to the original 
service so that the public will be able 
to receive the same service on their 
existing equipment as they did before 
without degradation. It is for this rea- 
son that color television had to de- 




WORAM AUDIO ASSOCIATES 

Consultants in Studio Systems 
Engineering, Design and Installation 

— offering — 

A COMPLETE CONSULATlON 
SERVICE FOR STUDIO 
PLANNING AND 
CONSTRUCTION 

FREE-LANCE RECORDING 
SERVICE IN THE 
NEW YORK AREA 

212 673-9110 
64 University Place 
New York, N.Y. 10003 





1 




4wM 


\ 


;%SUB-CARRIER^ 
mSlDEBANDSj^ 





0 50HZ I5K I9K 23KH2 39KHZ 



L 



STEREO COMPOSITE SIGNAL 



53KH2 



J 



Figure 3. The composite signal 
modulates the transmitter. It is 
composed of many signal elements 
on into the supersonic region. 



velop a signal compatible with black 
and white receivers; for the same rea- 
son, quad, or 4-channel stereo, is go- 
ing through the same throes. 

A matrix works in this manner. The 
output of the station's left and right 
audio channels terminates at the left 
and right input of the stereo genera- 
tor, where the signals go directly to 
the matrix. The matrix adds the left 
and right channels to produce a SUM 
(L-l-R) signal. At the same time, 
another part of the matrix inverts the 
right channel and combines it with 
the left channel to produce a difference 
(L— R) signal. The sum signal pro- 
vides the compatible monaural signal 
for mono sets. The difference signal 
will then amplitude modulate a 38 kHz 
subcarrier, producing double side- 
bands. The 38 kHz carrier itself is 
suppressed and only the sidebands 
remain. 

A synchronous detector is required 
in the receiver to recover these side- 
bands; this must be phased with the 
original carrier. The basic oscillator is 
a crystal-controlled 19 kHz oscillator 
in the stereo generator. The second 
harmonic (38 kHz) of this oscillator 
is modulated as the sub-carrier. The 
19 kHz signal itself is transmitted to 
synchronize the receiver detector. 

The composite output of the stereo 
generator has a bandpass extending 
from 50 Hz on up into the supersonic 
regions. It is made up of the L -I- R 
signal in the audio band of 50 Hz — 
15 kHz, a 19 kHz pilot signal, and 
double sidebands of the suppressed 
38 kHz carrier that extend from 23 
kHz to 53 kHz. This is the signal 
which modulates the transmitter and 
it will be the signal that is detected in 
a wideband demodulator in the re- 
ceiver. It must be further processed 
by a synchronous detector to recover 
the L — R signal, and along with the 
L-l-R signal sent into another matrix 
that will restore the original left and 
right audio channels. 



www.americanradiotiistorv.com 




LEFT 



LEFT AUDIO 
• 

INPUT 

^ ■ 

RIGHT AUDIO 



MATRIX 




MONAURAL 
MODULATION^ 
(L-l-R) 

Output 



(L-R) 



TO 

SUB-CARRIER 



OdB — 
2dB — 


J 

1 


._ 

RIGHT 




0 

2 


50HZ 




|2K I5KH7 



POOR 
SEPARATION 



(A) 



Figure 4. When audio input signals are 
180 degrees out of phase, the matrix 
will shove all the audio into sub-channels. 



POOR SEPARATION 

The matrix system relies very heav- 
ily upon phase relationships through- 
out. Anything, from the original mic- 
rophones to the ftnal destination in the 
receiver, which can distort the phase 
relationships can reduce the system's 
ability to recover and restore the orig- 
inal channels and their original in- 
tegrity. 

In the stereo generator, the sub-car- 
rier and pilot must be properly phased, 
the tran.smitter working properly, with 
no high standing waves on the trans- 
mission line or antenna problems. 
Once ail these have been originally 
adjusted properly, they generally re- 
main stable, unless some component 
fails (such failures usually trip out 
circuit breakers or alarms and must 
be corrected). From an operational 
standpoint, the problems which most 
beset the stereo system are in the 
audio system itself. These occur in two 
general categories — phase and ampli- 
tude response of the left and right 
audio channels. When phase is wrong, 
the two signals do not reach the mat- 
rix at the same time or the polarity 
of one channel is reversed. And when 
the audio response curve of each 
channel is not ideiuical, the varying re- 
sponse amplitudes do not permit com- 
plete cancellations in the matrix, so 
what remains shows up in the opposite 
channel and separation suffers. 

Polarity reversal of one channel 
( 180 degree phase shift) which places 
the two channels out of phase will 
cause the input signals to be shoved 
into the sub-channel and little on the 
main channel. This can be demon- 
strated by feeding a sine wave tone 
out of phase to both inputs of the 
stereo generator. With a sine wave, 
complete matrix action takes place and 
there is no signal on the main chan- 
nel; it is all in the sub-channel. With 
program, the mono receiver would 
suffer a severe drop in signal level. 

Assuming the original installation 
was correctly phased, reversal of po- 
larity usually happens because a patch 
plug has been turned over, or the wir- 
ing has been put back on incorrectly 



LEFT 




50Hz 8K I5K 

IB) 



Figure 5. Response curves must be 
identical. In (A) both curves are good 
individually but not identical. Poor 
separation will result at 15 kHz. In (B) 
both curves are poor, but identical. The 
system will have good separation. 

during maintenance, such as replacing 
the head on a tape machine. 

Phase shifts of less than 180 de- 
grees cause a lead or lag between the 
audio channels' phase relationships. A 
major cause of this is the path length 
of each channel. Signals starting at 
the microphone together should reach 
the matrix input at the same time. If 
they do not, complete matrix action 
cannot take place, so the unprocessed 
part of one signal will show up in the 
other channel and separation will de- 
teriorate. 

Anything which can cause one chan- 
nel to lead or lag behind the other 
channel will change the correct phase 
relationship. This can be caused by 
faulty or defective components in the 
audio system, but it can also be due 
to the original installation, where wir- 
ing lengths were not given careful con- 
sideration. All these differences are 
cumulative and a fixed, but incorrect, 
phase relationship is set up. There can 
also be other path length problems 
with Telephone Company lines to the 
transmitter site or when stereo re- 
motes are done over Telephone Com- 
pany lines. 



AMPLITUDE RESPONSE 

The response curve of each chan- 
nel must be identical with the other, 
or proper matrixing cannot be done, 
and what is left uncancelled will show 
up in the other channel. Separation 
does not essentially have a relation- 
ship to fidelity, but instead, to identi- 
cal response curves, whether these are 
good or poor, assuming there is no 
phase shift also involved. For exam- 
ple, each channel has a reasonably 
good response curve, but not identical 
to the other. One is flat all the way to 




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RAMKO RESEARCH 

3516 C LaGrande Blvd. 
Sacramento, California 98523 
Telephone (916) 392-2100 

Circle 31 on Reader Service Card 



www.americanradiohistorv.com 



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FLAT BASELINE 



GOOD SEPARATION 
(A) 



BASELINE 
NOT FLAT 




NORMAL 
BASELINE 



POOR SEPARATION 
(B) 



F/gure 6. Use an oscilloscope to 
measure the composite signal out of 
a stereo generator. 



15 kHz. The other one is flat out to 
12 kHz but then rolls off 2 dB at 15 
kHz. Both curves are in specs as indi- 
vidual curves go, but there will be 
poorer separation above 12 kHz. On 
the other hand, two identical but poor 
response curves, for instance, flat to 
8 kHz and then rolling equally so the 
response at 15 kHz is down 10 dB, 
will have good separation. 

Many, many faults or misoperations 
along the way can effect the response 
curve of the channels. There may be 
improper alignment of a tape machine 
head, a defective stylus in a turntable, 
improper level settings of amplifiers, 
impedance matching problems, misad- 
justed equalizers etc. Anything which 
can effect system response, unless it 
effects both cimnnels equally, will show 
up as poor separation. 

MEASUREMENT 

We can listen to the signal off the 
air with a good receiver and obtain a 
qualitative measurement of the sepa- 
ration, but this does not tell us how 
much separation is present. To mea- 
sure this, a sine wave generator and 
the modulation monitor can produce 
the information. Use the method de- 
scribed in the instruction manual for 
the monitor, but feed the signal to the 
input of the audio system to get the 
real separation figure. All this assumes 
that the monitor is properly adjusted 
and its own separation and phasing 



are correct. If the monitor is incor- 
rect and the system adjusted to read 
correctly on the monitor, the system 
would be actually misadjusted, even 
though it would appear correct on the 
monitor. 

To verify the monitor figures, feed 
a sine wave to the input of either the 
left or right channel and measure the 
output of the stereo generator com- 
posite signal with an oscilloscope. The 
base line on the scope figure should 
be flat or nearly so. Next, check the 
output of the detector in the monitor 
(the composite signal) and note the 
flatness of the base line. This check 
will measure the signal after it has 
passed through the transmitter. As- 
suming that the pilot phasing was cor- 
rect, if there is not a very flat base 
line, tweak up the stereo generator ad- 
justments. If this flattens the base line 
out at the output of the stereo gen- 
erator but not much out of the moni- 
tor, there are some transmitter prob- 
lems. But if it doesn't flatten out the 
base line after the stereo generator, 
then there are some audio system 
problems. In most cases, this is where 
the problem will be. So, you will have 
to go to work on the audio system, 
but look first for audio response prob- 
lems. ■ 



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is: 




■1 jI\ 



If you work ^tfa 
microphones, 
you need this book! 




Ihemosthnportant 
microphone book 
everpublished. 

A practical, non-theoretical reference manual for those 
involved in the application of microphones for tv, motion 
pictures, recording and sound reinforcement. 

At last, the practical aspects of microphone design and 
application have been prepared and explained in one 
concise, fact-filled volume by one of audio's outstanding 
experts. This book is so full of useful information, we think 
you'll use it every time you face a new or unusual 
microphone problem. 

Perfect for Reference or Trouble-shooting 

The twenty-six fact-packed chapters in this indispensable 
volume cover the field of microphones from physical 
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evaluation to applications, accessories and associated 
equipment. Each section is crammed with experience-tested 
detailed information. Whatever your audio specialty you 
need this book! 

Along with down-to-earth advice on trouble-free 
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dozens of invaluable secrets learned through his many years 
of experience. 

He solves the practical problems you meet in day-to-day 
situations. For example: 

* When would you choose a cardioid, omni-directional, or 
hi-directional mic? 

* How are omni-directional mics used for orchestral pickup? 

* How does dirt in the microphone rob you of response? 

* How do you space your microphones to bring out the best in 
each performer? 

This text is highly recommended as a teaching tool and 
reference for all those in the audio industry. Price: $20.00 



THE AUTHOR 

Holder of twenty-three patents on electro-acoustic products, 
Lou Burroughs has been responsible for extensive contribu- 
tions in the development of the microphone. During World 
War II, he developed the first noise cancelling (differential) 
microphone, known as the model T-45. Used by the Army 
Signal Corps, this achievement was cited by the Secretary of 
War. Burroughs was the creator oi acoustalloy , a non-metallic 
sheet from which dynamic diaphragms are molded. This ma- 
terial made it possible to produce the first wide-range uni- 
form-response dynamic microphone. Burroughs participated 
in the design and development of a number of the micro- 
phones which have made modern broadcasting possible ~ the 
first one-inch diameter wide-range dynamic for tv use; the 
first lavalier; the first cardiline microphone (which ultimately 
won a Motion Picture Academy award) and the first variable- 
D dynamic cardioid microphone. He also developed the first 
wind screens to use polyester foam. Burroughs was one of the 
two original founders of Electro- Voice, Inc. He is a charter 
member of the Society of Broadcast Engineers and a Fellow 
member of the Audio Engineering Society. 



ORDER FORM 



Sagamore Publishing Co., Inc. 

1 120 Old Country Road, Plainview, N.Y. 1 1803 

Please send [ ] copies of MICROPHONES: DESIGN AND 
APPLICATION at $20.00 each. 



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l)new ptoductsAsenHces 



ELECTRONIC CROSSOVER 



CD 



o 

CO 



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CM 
CM 



• Model X0312 keeps all outputs in 
phase at all frequencies and provides 
continuously tuneable crossover fre- 
quencies from about 100 to 1,000 Hz 
and 1,000 to 14,000 Hz. The manu- 
facturer claims distortion of less than 
0.1 per cent and a signal-to-noise ra- 
tio of better than 80 dB. High- and 
low-pass filters in each crossover are 
permanently cro.ssed at 3 dB down. 
The state-variable filter gives a 12 dB 
per octave Butterworth response. The 
sum signal is fiat ± 1 dB from 20 to 
20,000 Hz. The crossover has bal- 
anced 600-ohm transformer outputs 
individually adjustable to a maximum 
of 8 volts. Voltage gain is adjustable to 
a maximum of 2. 

Mfr: Stevenson (Interface Electronics) 
Circle 50 on Reader Service Card 

OSHA MEASUREMENT SET 




• Sound level measuring set model 
1983 meets ANSI S1.4 1971 Type S2A 
standards. The unit spans a single 
range, 70 to 120 dBA. Operation of 



the meter is quite simple, with no 
range or weighting selections to be 
made. The meter spans the 50 dB 
range in clearly marked 1 dB incre- 
ments; OSHA limits are printed on 
the meter face. In addition to the 
meter, the set includes a sound-level 
calibrator, windscreen, carrying case, 
etc. 

\ffr: GenRad, Inc. 

Circle 51 on Reader Service Card 

FOUR-WAY LOUDSPEAKER 



SOUND ABSORBING FABRIC 



0 



• High sound pressure levels with 
moderate power input was aimed at in 
the design of model 7 loudspeaker. 
Particular emphasis is placed on trans- 
mission of upper bass and lower mid- 
range tones with fidelity. The loud- 
speaker employs six drivers. It has a 
filter network rather than a conven- 
tional crossover, which the manufac- 
turer claims offers improved transient 
response and greater transparency. The 
speaker has been designed to handle 
the special needs of rock music, as 
well as other musical forms. 
Mfr: Rectilinear Research Corp. 
Price: $399. 

Circle 52 on Reader Service Card 




Audio Level Optimizer 

Maximizes average program level, restricts instantaneous peaKs. Independent 
peak limiting and gveragc compression fully gated to minimize ' brcathmti 
and ■pjnping ' Frequency- selective limiting gpugn for FN* available. 



1 



I N Q VO N I G S Eiecitonics tof Racortiin^ and Broadcasting 




• Sheerfill 3A soundproof fabric is 
lightweight, flexible, and translucent. It 
can be easily cut and draped. The 
coated beige surface is easily washable 
and fire-resistant. Acoustical proper- 
ties test out to NRC 0.60 minimum, 
ASTM-C423-66. The manufacturer 
claims a tensile strength of 200 mini- 
mum lbs./ in. A thin fabric, Sheerfill 
combines with air in front and in back 
of the installation to furnish sound ab- 
sorption. 

Mfr: Chemical Fabrics Corp. 
Circle 53 on Reader Service Card 



LIVE PERFORMANCE MIXER 




• This fifteen-microphone channel 
mixer provides treatments for left and 
right p.a. and stage monitor with two 
auxiliary outputs. Each input channel 
has tone controls for bass lift or cut, 
treble lift or cut, and middle lift; a 
fourth tone control, continuously va- 
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mum/minimum lift. There is a con- 
tinuously variable sensitivity control, 
switched input attenuator and a peak 
reading meter. Rotary or linear faders 
control monitor output and treatment 
channels. The device features inde- 
pendent two-track tape recorder level 
controls, listen and talk facilities, and 
two auxiliary peak reading meters. 
Five input channels are built as a sin- 
gle module and the output and aux- 
iliary channels are built as another 
module of the same size. 

Mfr: RSE (Lamb Laboratories) 
Circle 54 on Reader Service Card 



Circle 35 on Reader Service Card 



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16MM TAPE RECORDER 



ECONOMY SPEAKER SYSTEM 



FUNCTION GENERATOR 




• A lape drive system using a smooth 
eapslaii, claimed lo give very short 
siari-tinies, sjieed staliilily, low wow 
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Featured in DS-ld 16mm. pcrloratecl 
tape recorder/ reprotlucer. Synchroni- 
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pability, tape search aiul ail operation- 
al ("eaiines to meet current and pro- 
posed rccoiding sUuulards. The unit is 
suitable for liliii tiuhbing as well as for 
straight record/ replay. 
^^^)^: Scliliunhcri;er i iisinnnenis 
Circle 55 on Reader Service Card 




• Techniques for handling crossoveis 
and in balancing phase lags and leads 
of multiple drivers, relined in the de- 
signing of this manidaclurer's high- 
priced speakers have been irsed in the 
creation of economy 3-way Monitor 
Jr. All drivers (12-in. transmission- 
line woofer, 1'2 in. dome midrange. 
l-in. dome tweeter) deliver temporal 
information piecisely in phase. The 
manufacturer claims a dimensional 
quality to the sound, repioducing or- 
chestral sounds in relation to the in- 
struments" position — left, right, front. 
Or rear. Available in bookshelf or 
pedestal models. 
A//;.- Infniiiv S\ \lein'i. Inc. 
Price: $225. 

Circle 56 on Reader Service Card 




• Nine modes of operation are pos- 
sible with model 5300 function gener- 
ator. In addition to separate wave- 
forms and ramp outputs, pulse, sweep, 
and burst modes, it olTers an exponen- 
tial ramp function for logarithmic 
sweeping. The exponential sweep in 
conjunction with the linear sawtooth 
output enables semilog plotting. There 
is external voltage control of main out- 
put frequency. Pulses can be as nar- 
row as 200 ns at rep-rates anywhere 
between tOO kHz and 0.1 Hz. An ad- 
justable trigger gives a one shot per- 
formance of either a single cycle of a 
waveform or a single frequency sweep. 
Mfr: Krolut-Hile Corp. 
Price: $695. 

Circle 57 on Reader Service Card 




ULTIMATE 




This system, 
used forthe 1975 
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represents the 
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MARC SAUL 

Understanding 
Harmonic Distortion 

Harmonic distortion generated by audio equipment discolors 
musical output, sometimes objectionably. Testing and 
making the necessary adjustments will keep the distortion 
to a minimum. 



HARMONICS, which are multiples of a funda- 
mental frequency, are what give music and 
speech its particular character and timbre. 
Without harmonics, music and speech would 
sound dull and lifeless and it would be difficult to distin- 
guish one voice or musical instrument from another. 

For example, assume we strike the low-C note on a con- 
cert grand piano. In addition to the fundamental sine-wave 
frequency of 32.7 Hz being produced, harmonics of up to 
about the fiftieth of the fundamental tone will be gen- 
erated. Furthermore, since the piano sound board is not 
large enough to radiate frequencies much below 50 or 
60 Hz, the fundamental may be missing altogether. The 
output waveform then consists almost wholly of the har- 
monics (see FiGURK I). Without these harmonics, and 
particularly the higher order harmonics, the note would 
practically disappear or would sound muffled and without 
a distinct piano character. 

Audio equipment, such as line or monitor amplifiers, 
recording or playback amplifiers, tuners or receivers, and 
cutting heads or loudspeakers, are not musical instru- 
ments. This equipment must not introduce harmonics of 
their own so that they color the tone of the instruments 
or voices being handled. Instead they must be nearly 
perfectly transparent as possible to the sound signals be- 
ing amplified or being transduced. By the extent that they 
introduce their own harmonics or other signals, they 
produce distortion. 

Since no piece of audio equipment is perfectly distor- 
tionless, some distortion must be tolerated. The idea. 



Marc Saul has for many years been a writer and 
editor, covering the audio scene. 




Figure 1. Waveform of a low note struck on a piarjo. 



though, is to have as little distortion as possible so that 
the audio signals being handled are as free from alteration 
as possible. With good equipment, the amount of distor- 
tion will be below the level at which it can be perceived. 



HARMONICS AND NON-LINEAR DISTORTION 

Audio equipment is subject to several different types of 
distortion. But one of the most important is harmonic dis- 
tortion. This occurs when the equipment being used 
changes the waveforni of the signal being handled in the 
same way that it would be changed if harmonic frequen- 
cies were added to it. It also occurs when the equipment 
alters the size and shape of the harmonics already in the 
signal by either boosting or attenuating these harmonics. 

Now consider a perfectly pure 400-Hz sine-wave sig- 
nal being applied to a recording amplifier. If the amplifier 
has no harmonic distortion, then the output waveform 
would be a magnified but otherwise wholly unchanged rep- 
lica of the input (Figure 2A). However, if the outputs 
are as shown by the solid-line waveforms of Figures 2B 
through 2H, then harmonic distortion is present. The 
amplifier acts as though it were adding additional fre- 



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(fl) 

400 Hz INPUT a 
PERFECT OUTPUT 



(B) 

OUTPUT WITH 
2ND HARMONIC 



(C) 

OUTPUT WITH 
3RD HARMONIC 



(D) 

OUTPUT WITH 
4TH HARMONIC 




(E) 

OUTPUT WITH 2ND, 
3RD, 4TH HARMONICS 



{ ^ 



J 



:f) 

OUTPUT WITH LARGE NO OF 
ODD HARMONICS 




(G) 

OUTPUT WITH LARGE NO. OF 
ODD a EVEN HARMONICS 




(H) 

OUTPUT WITH LARGE NO. OF 
ALL HARMONICS WITH RANDOM 
AMPLITUDES a PHASES 



Figure 2. Waveforms with various riumbers of harmonics. 



qiiencies which, when added lo the fundamental, result in 
the distorted waveforms shown. 

With second-harmonic distortion, the sine wave takes 
on a skewed appearance and the downward slope has a 
couple of ripples in it. With third-harmonic distortion, the 
peaks of the waveform are changed into dips. When fourth- 
harmonic distortion is present, both dips and ripples ap- 
pear. In a case where there are a very large number of 
odd, in-phase harmonics the waveform takes on a square 
appearance (Figure 2F). Hence, if the amplifier clips 
both positive and negative peaks of an incoming sine wave, 
the effect is as though a large number of odd harmonics 
have been added. With a large number of all harmonics, 
the waveform is converted into a sawtooth (Figure 2G). 
Finally, with numerous harmonics having random ampli- 
tudes and phases with respect to the input, the irregular 
output waveform in Figure 2H would result. 

Next, consider the input-output linearity on the transfer 
characteristic of a piece of audio equipment, such as a 
playback amplifier. If the amplifier were perfectly linear, 
its transfer characteristic would be a straight line as shown 
in Figure 3A. With a sinusoidal input voltage, the output 
voltage would be a replica of the input — also sinusoidal. 

If the amplifier has a transfer characteristic that is non- 
linear, as shown in Figure 3B, the output would have a 
positive peak that is flattened out while the negative alter- 
nation is normal. The result is a disorted output waveform. 
A waveform such as this, with positive and negative half 
cycles of difl:"erent shapes and areas along with a steady 
(rectified) d.c. component, which in this case is negative, 
has evi'/i-harmonic distortion. 

With a different type of nonlinearity, as shown in Fig- 
ure 3C, the S-shaped transfer characteristic results in an 
output waveform that is tall and peaky. The result again 
is distortion. A waveform such as this with positive and 
negative half cycles similar in shape has ot/J-harmonic 
distortion. 

SINGLE-ENDED AMPLIFIER 

In a single-ended amplifier, the harmonics that are gen- 
erated are mainly even harmonics. On the other hand, a 
push-pull output stage usually operates in such a way 
that the transfer curve of one transistor in the output 
stage overlaps and cancels out the non-symmetrical non- 



linearities in the other transistor of the push-pull stage. As 
a result, the even harmonics are largely canceled out. 
Most of the distortion then consists of odd harmonics 
alone. 

Some harmonics are more displeasing to listeners than 
are other harmonics. In general the lower-order harmonics 
(say the second through the fifth) result in tones that are 
on the musical scale; hence they are not unpleasant to hear. 
On the other hand, the higher-order harmonics (say the 
seventh through the twenty-fifth) are mostly not on the 
musical scale and are decidedly unpleasant to listen to, 
even when the harmonics are fairly low in amplitude. 

For example, assume we apply a 250 Hz sine wave to an 
audio system. If the system introduces harmonic distortion, 
we will find that the second through the sixth harmonics 
are musically related to the fundamental, hence they are 
not unpleasant to listen to although they certainly consti- 
tute distortion because they were not present in the input 
waveform. Additional musically related harmonics include 
the eighth, tenth and twelfth, as well as the sixteenth, twen- 
tieth and twenty-fourth. Non-musical, dissonant harmonics 
are the seventh, ninth, eleventh, thirteenth, fourteenth, fif- 
teenth, seventeenth, eighteenth, nineteenth, twenty-first, 
twenty-second, twenty-third and twenty-fourth. 

Although practically no music produced by acoustic 
rather than electronic musical instruments and practically 
no speech is purely sinusoidal, or lacking in harmonics, we 
do not want our electronic audio equipment to generate 
the harmonics. The job of the equipment is to duplicate 
the original input without introducing any harmonics of 
its own. The perfect amplifier, then, is one that repro- 
duces exactly the waveform, no matter how complex, that 
is applied to it. 

TOTAL HARMONIC DISTORTION 

The harmonic-distortion factor of a signal is the ratio 
between the total rms values of ail the harmonics to the 
total rms value of the fundamental plus all the harmonics. 
Expressing this factor as a percentage (multiply the fac- 
tor by 100) gives us a measure of the percentage of total 
harmonic distortion (thd). 

To be more exact, the percentage of thd is equal to the 
square root of the sum of the squares of all the harmonics 
divided by the square root of the sum of the squares of the 



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fundamental and the harmonics, all multiplied by 100. 

Assume we have a distorted waveform (fimdamental 
plus harmonics) with an rms value of 50 volts and we find 
that we have, in addition to the fundamental signal, a sec- 
ond harmonic of 2 volts and a third harmonic of 1.5 volts. 
Our percentage of thd is the square root of 2- plus 1 .5- or 
2.5 divided by 50, all multiplied by 100, or 5 per cent thd. 

Sometimes the distortion is weighted in proportion to 
the order of the harmonics. When this is done, the per- 
centage of the individual harmonics is multiplied by a 
weighting factor that increases as the order of the har- 
monic increases. 

With just about every system of amplification other than 
class B, the percentage of total harmonic distortion de- 
creases as the power output level is reduced. In addition, 
as the output power level is reduced the percentages of 
the higher order harmonics decrease more rapidly than 
those of the lower order harmonics. This means that the 
thd usually decreases as the power output is reduced. How- 
ever, in some transistor amplifiers, when you go down to 
very low output powers, thd may actually begin to rise 
again slightly. 

When negative feedback is used, all the harmonics are 
reduced in the same proportion. This does not affect their 
relative importance, except when the overload point is 
reached. 

HOW MUCH DISTORTION? 

An important question is just how much distortion we 
can perceive or tolerate in an audio system. We can, in 





ATTEN 




AUDIO 






GENERATOR 


X ^ 






(y) Ml V 










HARMOMC 
DISTORTION 
METER 






: 


JLOAD 



Figure 4. Test setup used to measure total harmonic 
distortion of amplifiers. 



general, tolerate less distortion with music than with speech. 
Also, as we increase the range of frequencies that our sys- 
tem covers, we can tolerate less distortion. This means 
that in a fairly restricted bandwidth system, say covering 
a frequency range of 100 to 5,000 Hz, we can tolerate far 
more distortion than in a wide-band audio system that 
covers from 20 to 20,000 Hz. The narrow-band system 
simply does not respond to the higher order harmonics. 
However, the disadvantage of the narrow-bandwidth sys- 
tem is that it does not respond to the desirable and neces- 
sary very low and very high frequencies. Hence, the price 
that we have to pay for increased frequency coverage is 
that we must exert more effort to acquire lower distortion 
in the system. 

It is difficult to set down specific limits for the total har- 
monic distortion percentage. This is because the thd usu- 
ally lumps together all the harmonics and does not specify 
which harmonics are involved. 

As an example, suppose we consider a waveform with a 
thd of 5 per cent, as calculated above. Nothing is usually 
said about whether this percentage represents mainly odd 
harmonics, even harmonics or a combination of both odd 
and even. Further, if the harmonic distortion consists of a 
number of harmonics, as is usually the case, nothing is 
indicated in the percentage figure to tell us the relative 
amplitudes of the various harmonics making up the distor- 
tion. Any of these conditions would produce a differently 
shaped distorted waveform and a different effect on the 
listener. 

In general, listeners will tolerate a much larger amount 
of even-harmonic distortion than odd-harmonic distortion. 
This is because the even harmonics are largely musical 
and nondissonant, while the odd harmonics are not musical 
and are dissonant. Some circuits and even some transistors 
are more prone to emphasize certain harmonics than 
others. As mentioned above, harmonic distortion in most 
push-pull stages is largely odd harmonic, the even har- 
monics being canceled out. 

Because of these and other variables, the thd certainly 
does not tell the entire story of system performance. How- 
ever, it does provide us with a simple, convenient, and 
easily duplicated test that allows us to compare one with 
another. 

A number of tests were conducted some years ago by 



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It's easy to claim a NATURAL SOUND reverberation chamber. 



Producing one is something else. 



NATURAL SOUND is truly ^NATURAL' only if the chamber: 

• Includes a built-in NATURAL-length time delay- 
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• Creates an initial group of NATURAL-type first-order 
echos followed by randomly patterned diffusion of the 
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• Provides true NATURAL-stereo perspective outputs, 
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Only the Master-Room^^ series meets 
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a. 



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BAND- REJECTION 



V &0-80dB 



a. 



60- SC' SB 



l/3f l/2f f 2f 3f 4f 5f 
FREQUENCY 
(A) 



l/3f l/2f f 2f 3f 4f 5f 
FREQUENCY 
(B) 

Figure 5. The two types of response curves used in 
harmonic distortion analyzers. 



P 4 

cr 

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CO 

S3 



cr 



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10 20 30 40 50 
POWER OUTPUT -WATTS 



60 



70 



Figure 6. Distortion characteristic of a typical hi fi 
power amplifier. 



Dr. Harry Olson of RCA using single-ended, low-power 
(3 watt) triode and pentode tube amplifiers. The tests were 
conducted in a typical living room environment with a 
noise level of about 25 dB. A limited number of critical 
observers were used to rate the intentionally introduced 
distortion from objectionable — through tolerable — to per- 
ceptible. 

The results of these tests are the following: For an am- 
plifier high-end cutoff of 7,500 Hz, objectionable distor- 
tion occurred at 4 to 4.8 per cent thd for music and 6.4 
to 6.8 per cent thd for speech. Tolerable distortion oc- 
curred at 3.2 to 4.4 per cent for music and 4 to 4.8 per 
cent thd for speech. Perceptible distortion occurred at 
0.95 per cent thd for music and 1.15 to 1.2 per cent thd 
for speech. 

Next, the high-end cutoff was extended to 15,000 Hz. 
Under these conditions, objectionable distortion was 2,0 
to 2.5 per cent for music and 3.0 to 4.4 per cent for 
speech. Tolerable distortion occurred at 1.35 to 1.8 per 
cent for music and 1.9 to 2.8 per cent for speech. Per- 
ceptible distortion occurred at 0.7 to 0.75 per cent with 
music and 0.9 per cent with speech. 

Other tests, made with telephone line equipment by the 
British Post Office in conjunction with the BBC, disclosed 
the following results for just detectable second- and third- 
harmonic distortion: For second-harmonic distortion, up 
to 25 per cent below 100 Hz, up to 3 per cent below 200 
Hz, up to 1 per cent below 400 Hz, and below 1 per cent 
above 400 Hz. For third-harmonic distortion, just detec- 
table distortion was up to 5 per cent below 100 Hz, up to 
2 per cent below 200 Hz, and up to 1 per cent above 
400 Hz. 

The better the equipment, the less thd it will have. A 
perfect amplifier would have zero per cent total harmonic 
distortion. Since such an amplifier does not exist, we will 
have to settle for a thd figure below 1 per cent for very 
good performance and below 0.5 per cent for exceptional 
performance. There are a few amplifiers available whose 
^ thd approaches or is lower than the residual distortion in 
the instruments being used to measure the thd. 

o 

^ MEASURING THD 

^ In order to measure total harmonic distortion we can 

use the test setup shown in Fiqure 4. A jow-distortlpn 
aitdio generator, whose output is monitored with an ex- 

?3 ternal meter Ml (if the generator itself does npt have 



such a meter), is applied through an attenuator to the in- 
put of the audio unit under test. In this case, we are show- 
ing an amplifier being tested. The output of the amplifier 
is monitored by a second meter, M2. The amplifier is also 
properly terminated by a load resistor as shown. The am- 
plifier output is applied to the input of the harmonic-dis- 
tortion meter. This meter, or analyzer, will read the per- 
centage of thd directly 

The harmonic-distortion meter contains a selective audio 
voltage amplifier, with adjustable attenuation, whose output 
is connected internally to a high impedance vacuum tube 
or solid-stage voltmeter circuit. The purpose of the selec- 
tive amplifier is to suppress or null out the fundamental 
frequency of the audio generator so that a measurement 
can be taken of the remaining harmonics. 

The usual method of obtaining this selectivity is by the 
use of a tunable Wien-bridge or bridged-T network that 
is used to put a sharp notch in the instrument's response 
curve (Figure 5A). When the notch is adjusted to the 
fundamental frequency of the audio generator, the funda- 
mental frequency is effectively removed or suppressed by 
60 to 80 dB. The meter in the harmonic-distortion analyzer 
now has applied to it all the other components of the wave- 
form. These are mainly harmonics, but also included is any 
hum or noise produced by the unit under test. 

When the instrument is used to take a measurement of 
thd, the selective amplifier is first bypassed entirely and a 
meter reading is taken of the output of the amplifier un- 
der test. This reading is the value of the fundamental fre- 
quency plus the distorting harmonics. The meter is now 
adjusted for full-scale (100 per cent) reading. Now the 
selective amplifier is switched in and it is carefully bal- 
anced to suppress the fundamental. This is done by ad- 
justing the notch frequency of the selective amplifier for 
a null or minimum reading. Now the residual meter read- 
ing is a direct indication of the percentage of total har- 
monic distortion. 

Some harmonic-distortion meters employ sharp cutoff 
high-pass filters to eliminate the fundamental frequency 
(Figure 5B). With such a curve, not only is the funda- 
mental frequency removed but the hum and noise below 
the fundamental are also effectively eliminated. As such 
filters are not usually adjustable, these instruments may 
use a half dozen or so filters with different cutoff fre- 
quencies in order to permit measurements to be made at 
various fundamental frequencies. In some cases, two such 
filters are used, one cutting off at 400 Hz and the other at 
1,000 Hz, This permits thd measurements to be made at 



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90 ^ 


«o nj 


0 






•n .1 







GENEnATOR I 



Your new automatic 
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for balanced measurements 



REDUCED OPERATOR ERROR 

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• +26 to -90 dBni atfcnuator 

• distortion measurements to .002% 

• fast 5-sccond measuring speed 

• automatic nulling, optional auto- 
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• both harmonic and optional inter- 
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In the 1710A you get a transformer- 
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output from +26 to -90 dBm in 0.1 
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FAST, SIMPLE MEASURING 

Automatic nulling and the automatic 
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IM OPTION 

An additional optional bonus is that 
the 1710A also measures intermodula- 
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—1 

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Figure 7. Test set up for 
oscilloscope observation 
of harmonic distortion. 



AUDIO 
GENERATOR 



Ml 



ATTENUATOR 




AMPLIFIER 
UNDER 
TEST 



: LOAD 



e 



-o 

V 

SCOPE 



1 



these two fundamental frequencies. 

The measurements taken with these two types of har- 
monic-distortion meters will indicate two slightly different 
thd figures. The instrument with the band-rejection filter 
will usually yield slightly lower values of thd because its 
figures do not include the low-frequency noise and hum. 
In most cases it is the bridge-type unit that will be used. 

A number of years ago it was common to take harmonic- 
distortion measurements at only one or two frequencies 
around the middle of the audio band, such as 400 or 1,000 
Hz. Later, though, with improvements in audio gear and 
the emergence of really high-quality hi-fi consumer prod- 
ucts, many manufacturers were anxious to show just how 
good their equipment was. Therefore, it became common 
to make thd measurements throughout the entire range from 
30 to 15,000 Hz or even froni 20 to 20,000 Hz. Such 
measurements impose a severe test on a unit because it is 
far more ditlicult to handle the very low and very high 
frequencies with a minimum of distortion than it is to 
handle the frequencies in the middle of the audio range. 

It IS also common to make thd measurements over a 
wide range of output powers or output voltages, from 



some very low value up to and beyond the overload re- 
gion of the equipment under test. In general, as the out- 
put power or voltage is increased, so is the amoimt of dis- 
tortion. Usually, the increase is smooth and gradual up to 
the overload point, where there is a sudden increase in 
distortion. Ampliliers should be rated at a power or volt- 
tage just below this overload point, while the thd is still 
only a small value, say 1 per cent. 

In Figure 6 we see an amplifier's percentage of thd (at 
some mid-frequency) plotted against its output power. In 
this case, the amplifier has a thd of less than 1 per cent 
below output powers of 50 watts. At 50 watts, the thd 
is just 1 per cent. Above this power, the thd rises quickly 
to a vakie of 5 per cent at 60 watts. This amplifier would 
then be rated at 50 watts with 1 per cent thd. 

A high-quality 50-watt amplifier whose thd has been 
measured over the entire audio range may have the fol- 
lowing specification; 'Total harmonic distortion at 1 per 
cent or below from 20 to 20-000 Hz within 1 dB of 50 
watts." Such an amplifier would have no trouble at all in 
producing up to a full 50 watts of output at 1 per cent or 
less of thd over most of the audio range. At the very ex- 



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Figure 8. Distortion patterns on an oscilloscope being 
used to observe harmonic distortion. 



ircmcs ot the range, however, it would still be producing I 
per cent or less disioriion al powers up to 40 watts {which 
is I dH below 50 walls). 

Another itnit might be rated as lollows: "Total harmonic 
distortion below 2 per cent from 30 to 15,000." Such 
equipment mi^ht very well have a thd of a fraction of I 
per cent al 1,000 Hz, but distortion would not exceed the 
2 per cent figure at full rated power output over the fre- 
quency range specilied. 

USING A 'SCOPE TO OBSERVE DISTORTION 

An oscilloscope may also he used lo observe harmonic 
distortion in an amplilier or other piece of audio equip- 
ment. As a rule, it is dillicult to see distortion much less 
than 3 to 5 per cent on a 5-inch cathode-ray lube. In some 
cases, though, especially with higher order harmonics, 
some 2 to 3 per cent distortion can be observed. It is im- 
portant that the 'scope used have vertical and horizontal 
ampliiiers with similar or equal frequency responses and 
phase characteristics. 

To check distortion \vith a 'scope, use he setup shown 
in Figure 7. Here the audio generator is fed through a 
monitoring meter and an attenuator to the input of the 



amplifier under test. The output of the generator is also 
applied lo the horizontal-input terminals of the 'scope, 
whose horizontal sweep frequency is turned oil. The out- 
put of the amplilier imdcr test is monitored by meter M2 
and is properly terminated in a load resistor. This output 
!S also applied to the vertical-input terminals of the oscillo- 
scope. The frequency of the audio generator is usually set 
at some convenient middle frequency, such as 400 or 1,000 
Hz, or it may be set to some low frequency, such as 40 
or 50 Hz. 

The output of the generator is now increased from some 
very low value up to the point where the amplifier begins 
lo overload or up to the rated power output. 

The waveform seen on the 'scope will be a perfectly 
straight diagonal line, assuming that the 'scope's gain con- 
trols arc adjusted so that equal voltages are applied to the 
dellecting plates of the cathode-ray tube (Figure 8A). 
This straight line is actually the transfer characteristic of 
the amplifier under test. At and above the overload point 
the straight line will begin to show some curvature at 
either one or both ends, or it may show some curvature 
somewhere along the length of the Irace. If the curvature 
is at one end (Figure 8B), you are seeing the results of 
even-harmonic distortion, and if Ihe curvature is at both 
ends (Figure 8C), you are seeing the results of otld-har- 
monic distortion. 

A drawback of this technique is that you cannot readily 
determine the actual percentage of harmonic distortion. 
About al! that can be done is to determine whether or not 
distortion is present. The more the nonlinearity or distor- 
tion, the greater will be the curvature of the scope trace. 

A measurement of total harmonic distortion, although it 
does not tell the entire story about a unit's characteristics, 
is one of the most useful performance specifications that 
can be measured. ■ 





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HARALD BODE 



Frequency Shifters 
For Professionals 

Integrated into electronic synthesizers, frequency shifters 
increase the possibilities of tone interplay and innovation. 



IN CONTRAST to transposing devices, frequency shift- 
ers change the harmonic structure of any natural 
or synthesized sound received at the input, thus 
creating new sounds for the innovative user. Among 
the different types of frequency shifters known, the 
model 735 Bode Frequency Shifter and its counterpart, 
made by Moog Music, Inc. are the most versatile. In 
these devices, the amount of frequency shift is voltage- 
controlled according to one of two control modes. 

In the linear mode, the amount of shift is continuously 
variable from -|-5 kHz, through zero, to -5 kHz. In this 
mode of operation, an alternating control voltage intro- 
duces additional sidebands into the shifted outputs with- 
out actually changing the average amount of shift. In 
the exponential mode, the amount of frequency shift 
doubles for each one-volt increase in control voltage, 
thereby producing changes in the amount of frequency 
shift that run parallel to the frequencies of synthesizer 
oscillators and filters that are being controlled from the 
same voltage. The resulting effects and some interesting 
applications will be described in this article. 

Frequency shifters have been built for a number of 
purposes, such as the reduction of acoustical feedback 
(howl) in sound reinforcement systems and, in a multi- 
ple single sideband configuration, for the simulation of 
a choral tone effect. Whereas instruments of this kind 
use small frequency changes to achieve the desired re- 
sults, there is an apparatus used for substantial changes 
of musical frequencies. This is known in the German 
broadcasting system under the name Klangumwandler, 
which, directly translated, means sound converter. This 
device operates through double heterodyning and single 
sideband production through the use of a single side- 
band filter. 



Harald Bode is a well-known inventor of devices 
used in electronic music. He heads Bode Sound 
Company in North Tonawanda, N.Y. 



The techniques employed for frequency shifting are 
basically the same as known for single sideband produc- 
tion — heterodyning and the use of the phase shifting 
principle. 1 used a combination of both principles in a 
special frequency shifter built for the Electronic Music 
Centers of Columbia and Princeton universities in 1963." 

Since the introduction of this rather specialized in- 
strument, I have developed a .number of different mod- 
els. Among these is a carrier injection model of 1964, 
which subsequently was manufactured by Moog. In a 
more recent joint effort by Moog and myself, a versatile 
frequency shifter was developed and presented at the AES 
Spring convention in Los Angeles in 1972.' This model 
has, among other features, a built-in beat frequency 
quadrature oscillator (patented in 1974), which is volt- 
age controllable, including a linear-to-exponential inter- 
face, making this frequency shifter compatible with volt- 
age controlled synthesizer modules and functions. 

BEAT FREQUENCY QUADRATURE OSCILLATOR 

A basic (simplified) block schematic diagram of this 
frequency shifter is shown in Figure 1. Through the 
input terminal, the program signal is entered into two 
phase shifting networks, 0i and 0,, which produce two 
output signals with a 90 degree phase difference relative 
to each other". These phase-shifted signals are then fed 
to the first inputs of two four quadrant multipliers. The 
second inputs of the same multipliers receive two 90 de- 
gree out-of-phase signals from a beat frequency oscilla- 
tor, which is composed of a fixed (20 kHz) and a vari- 
able (15-25 kHz) oscillator. The fixed oscillator is fol- 
lowed by a gate, a low-pass filter (or resonance circuit) 
to secure pure sine waves, and by two phase shifters 0-, 
and 04, which produce two 90 degree out-of-phase out- 
puts (sine/cosine relationship). After mixing these two 
components of the fixed frequency with the variable fre- 
quency, two beat frequencies are obtained at the outputs 
of mixers 1 and 2, which again are in sine/cosine rela- 
tionship. At the mixer outputs, the 10 kHz low pass fil- 



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MULTIPLIER I 



SIGNAL IN ( 



THRESHOLD 
CONTROL 



T 

90° 




AMP RECTIFIER 



OSC 
20 KHz 



SWo 



GATE 



L-P 
20KHZ 



VOLTAGE 
FOLL 



INVERTER SUMMING 
OUTPUT 
AMPLIFIER I 



MULTIPLIER 2 




OUTPUT I 



OUTPUT 2 



OUTPUT 
SUMMING AMPLIFIER 2 



VOLTAGE 
FOLL. 

Figure 1. A block schematic diagram of 
frequency shifter with beat frequency 
quadrature oscillator. 



MIXER 
I 



L-P 
10 KHz 



MIXER 



— 2 



L-P 
10 KHz 



T" 

90° 

_L 



CONTROL 
VOLTAGE 
INPUTS 




SW, 



LIN/EXP 


EXP 


OSC 


INTERFACE 


0 


15-25 KHz 






LIN 





CONTROL LIN 
VOLTAGE 
INPUT STAGE 



ters arc provided to eliminate the high frequency com- 
ponents of the beat frequency oscillators. Through the 
use of dircct-couplcd circuitry, this oscillator operates 
with a constant amplitude from d.c. to the highest beat 
frequency. 

When displaying the two output components on the 
X and Y axis of an oscilloscope, a clean circle appears, 
which reverses its rotation when going through zero 
beats. 

Going back now to the four quadrant multipliers 1 
and 2: These produce two sidebands each with a sup- 
pres,sed carrier. The sidebands are made up of the beat 
frequency plus and minus the program frequencies re- 
ceived at the input. Due to the phase relationship be- 
tween the two multiplier outputs, one of the sidebands 
is cancelled when combining the two output signals (at 
the voltage follower outputs) through summing. When 
combining the inverted output signal of multiplier 1 with 
that of multiplier 2, the other sideband is cancelled. Thus 
the two opposite sidebands appear at outputs 1 and 2, 
which means that one output produces an up-detuned 
signal and the other a down-detuned signal. 

FREQUENCY-SHIFTED SIGNAL 

So far, I have discussed the basic performance of fre- 
quency shifting functions. Before going into a descrip- 
tion of some of the other features of this instrument it 
may be of interest to see what the analysis of a frequency- 
shifted signal looks like. 

As an example. Figure 2 gives a graphical display of 
the (irst five harmonics of a sound before shifting (orig- 
inal frequency spectrum) on the horizontal center line. 



AMOUNT OF 
FREQUENCY 
SHIFT 

-I- 



5f ^ / V /-^- Z 

Ml 100) ^ 7 7 7 /t 



+ 3f (6601 



^ — ^"^ — 7f 



<2Z0I 



FUNDAMENTAL X i- X Y 5TH HARMONIC 
/ \/ END 3RD 4TH X-p/ 



X / / / / 



^'^t 18801 K X 



/ 



/ 



/ 



"/ / yj II 100) 



(1320) 



(1760) 



III 001 



(44CI 



(15401 



(13201 



f(2201 2f(4401 3f(660)4f(880) .SflllOO) 
/T (440) / \ (880) \ y \ 




ORIGINAL 
FREQUENCY 
SPECTRUM 

~f (2201 

~ 5f (6601 
~ (6801 

^f(IIOO) X \/ DOWN SWFiED 
FRE(iUENClES 

\ \ 
5TH 4TH 3RD 2ND FUND 

INVERTED FREQUENCY SPECTRUM 
Figure 2. The change of harmonics through frequency shift. 



and after shifting above and below this center line. The 
approximate amplitude values are chosen for a square 
wave, a waveform which has a hollow, clarinet-like qual- 
ity because it has only odd harmonics. 

Let's assume for the sake of illustration that the fun- 
damental frequency / equals 220 Hz. Then the harmonic 
frequencies will be 2f = 440 Hz, 3f - 660 Hz, 4f = 
880 Hz, and 5f - 1,100 Hz. If we now shift the fre- 
quency up by +f, or 4 220 Hz (seen on the vertical 
scale), then all of the harmonics go up in frequency and 
the new, shifted frequencies can be found at the inter- 
section of the solid diagonal lines and the horizontal line 
identified by •*-/. In this ease, the fundamental of the 
original sound has changed to 440 Hz, the third har- 



Q. 



03 

o 



to 



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---- ^ ^1 © © 



F/g. 3. 7/je //-ont pane/ /ayouf of the Model 735 Bode 
frequency stiifter. 



monic to 880 Hz, and the fifth harmonic to 1,320 Hz. 

It can be recognized immediately that these new fre- 
quencies are the first three harmonics of a sawtooth 
wave (or stringlilce quality), or, if not extended, of a 
flute tone, with a fundamental one octave higher than 
that of the original. 

This is of course a very special example, which hap- 
pens to represent a frequency change by an amount that 
equals the fundamental frequency of the original sound. 
If, in contrast, the frequency shift is not related to the 
frequencies of the original spectrum, a new sound is pro- 
duced, the partials of which are no longer harmonically 
related. For instance, if the tone spectrum shown in 
Figure 2 is shifted by i-50 Hz, then the fundamental 
changes to 270 Hz, the second harmonic to 490 Hz, the 
third to 710 Hz, the fourth to 930 Hz, and the fifth har- 
monic to 1,150 Hz, and the original sound loses its 
identity. Sounds of bells, chimes, carillons, and the like 
fall into the category of tones with non-harmonic struc- 
tures. The frequency shifter is capable of producing an 
endless variety of sounds of this type. 

From the discussion of up-shifted sounds, the reader 
may derive as well the structure of down-shifted sounds. 
One interesting feature of the down-shifting is that, de- 
pending upon the amount of shift, part of the original 
spectrum or all of it is inverted, which leads to another 
family of interesting new sounds. 

In addition to the group of partials obtained from one 
of the outputs of the frequency shifter and represented 
by the solid lines in Figure 2, there are the partials de- 
rived from the complementary output of the instrument, 
represented by the dashed diagonal lines. 

EXPONENTIAL SHIFT CONTROL 

So far only a few examples for frequency shifting one 
single note have been discussed. But what if we have 
found an interesting sound and want to repeat it over 




m Figure 5. J tie auttior's syr\tttesizer, using frequency stiifters. 




Figure 4. The Moog version of Bode frequency 
shifter (tAodel 1630). 

the entire keyboard? This is accomplished with an ex- 
ponential shift control. 

Evidently the amount of frequency shift will need to 
be changed with the fundamental pitch so that the ratio 
of the shifting frequency versus the fundamental re- 
mains the same over the keyboard range. This will be- 
come obvious when cojisidering the first example, in 
which the amount of shift equalled the fundamental fre- 
quency. From this it follows that the amount of shift or 
the b.f.o. (beat frequency oscillator) frequency of the 
shifter has to move in the same musical intervals as the 
audio frequency fed to the signal input. Since the fre- 
quencies of the keyboard scale follow an exponential 
function, the same has to be true for the oscillator fre- 
quency of the shifter. 

For this reason, the linear voltage intervals of the key- 
board controller (or ribbon controller) of a synthesizer 
have to be translated into exponential intervals for the 
local oscillator of the frequency shifter, as shown in the 
diagram of Figure 1 (linear to exponential interface), 
just as it is done on voltage-controlled oscillators and 
other keyboard-controlled synthesizer modules. Through 
the inclusion of the exponential mode, the frequency 
shifter becomes a real-time performance instrument 
within a synthesizer installation. 

Another important feature is the variable sensitivity 
carrier squelch circuit, which eliminates the almost in- 
audible carrier feedthrough when the audio signal level 
at the input is below a preset threshold level. 

Figure 3 shows the front panel layout. The threshold 
control for the squelch circuit is seen on the left hand 
side. A light emitting diode above the control knob lights 
up when the incoming signal is above the preset threshold 
level. A mode selector and scale switch, under the head- 
ing. Scale, facilitates the selection of the exponential 
mode and the ranges from -t-5 to —5 Hz detuning 
through +5 kHz to —5 kHz detuning (linear), as well 
as calibration mode, which operates in conjunction with 
the zero adjust control. With this control, the instrument 
is initially calibrated to zero beat, indicated on the l.e.d. 
above the control knob. 

Using the main tuning knob (amount of shift control 
in the center of the instrument), the built-in beat fre- 
quency oscillatos. is either detuned in linear increments 
in accordance with the range selected on the scale switch 
or in exponential increments. In the latter case, the 
change by one dial increment corresponds to a one- 
octave frequency change. 

The mixture control facilitates the mixing of the two 
up and down detuned signals in any desired proportion. 



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SIGNAL 
INPUT 



AMP/RECT 



CARRIER 
INPUT 



GATE 



T 

90° 



T 

90° 



INVERTER 



MULTIPLIER I 



VOLTAGE 
FOLL. 



X4> 




MULTIPLIER 2 



VOLTAGE 
FOLL. 



OUTPUT 
AMP I 



-o OUTPUT I 



-O OUTPUT 2 



OUTPUT 
AMP 2 



Figure 6. A block schematic diagram of the carrier 
iniectior) type frequency shifter. 



In the center position, the output A + B equals the per- 
formance of a ring modulator. 

The inputs of the frequency shifter include one input 
jack for the program signal, signal in, and three input 
jacks for control voltages, control inputs. The outputs 
feed into two jacks each for one of the sidebands, Out A, 
two jacks each for the other sideband. Out B, and two 
jacks each for the mixture of both sidebands. 

On the right hand side, the line switch and the pilot 
light is shown on this particular model, which is equipped 
with a built-in power supply. 

Figure 4 shows the Moog version of the Bode fre- 
quency shifter, which fits into the modular assembly of 
the Moog synthesizers. All of the controls just explained 
(with the exception of the power switch and pilot light) 
can be found on the 1630 Moog model in a different 
geometric arningcment. Electrically both models are 
identical. 

A limited size custom synthesizer is shown in Figure 
5. Here the model 735 frequency shifter is in a case on 
the left hand side directly above the case with the Moog 
modules. 

OTHER TYPES OF FREQUENCY SHIFTERS 

Other types of frequency shifters include the hetero- 
dyning model, the carrier injection model, and models 
with a built-in c^uadrature oscillator. Of these, the latter 
two will be described briefly. 

A block schematic diagram for the carrier injection 
model (Bode model 750) is shown in Figure 6. Here the 
incoming signal is fed to two phase-shifting networks, 
0, and 0^, the output signals of which arc 90 degrees out 
of phase relative to each other over the audio range (35 
Hz to 16 kHz). The outputs of these networks are con- 
nected to the first inputs of multipliers 1 and 2, the sec- 
ond inputs of which receive their signals from two phase 
shifting networks, 0., and 0,, the basic circuit of which is 
identical to that of 0, and with the exception that they 
cover a frequency range from 8 Hz to 4 kHz. This Jai- 
ler range is more meaningful for frequency shifting car- 
rier frequencies. 

The phase filters 0., and 0, receive the carrier (usually 
a sine wave) through a gate, which is opened at a preset 
level of the program signal, so that there is no carrier 
feedthrough in the quiescent state. 



The output signals of multipliers 1 and 2 are summed 
at the voltage follower outputs to produce a frequency- 
shifted signal at output 2 in much the same way as it was 
described for the system in Figure 1. A signal of opposite 
shift direction is produced at output 1 by summing the 
inverted signal of multiplier 1 with the non-inverted sig- 
nal of multiplier 2. 

Figure 7 shows the front panel layout of the Bode 
model 750 carrier injection frequency shifter. The con- 
trols are, from left to right, the squelch threshold con- 
trol with the l.e.d. above the control knob, the sideband 
switch, which facilitates sideband reversal, and the mix- 
ture control for mixing of the up-detuned and the down- 
detuned signal in any desired proportion. If the propor- 
tions are equal, the signal at the A -r B output equals 
the performance of a ring modulator. 

The inputs of this frequency shifter include one input 
jack each for the program signal (audio in) and for the 
carrier signal (sine wave, 2 dBm nominal level). The 
outputs feed into two jacks each for the upper sideband 
(A Out, up-shifted signal), two jacks for the lower side- 
band {B Out, down-shifted signal), and two jacks for 
a mixture of both (A + B). 

In the installation of Figurk 5, this frequency shifter 
can be recognized on top of the instruments on the left 
hand side. 

The block schematic diagram of a further frequency 
shifter type with a quadrature oscillator for producing 
the frequency shifting sine/cosine signals is shown in 
Figure 8. From the preceding descriptions, this schema- 
tic should be self-explanatory. The Bode model 741 fre- 
quency shifter uses this system for feedback suppression 
in sound reinforcement systems. In this application, the 

o. 



• 


"6" 




m 




(<^t^ HS-S^ ' 





Figure 7. The fror)t panel layout of the carrier ^ 
injection-type Bode frequency shifter. w 



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MULTIPLIER 



SIGNAL 
INPUT 




SUMMING 

OUTPUT 



■^•^^ SIGNAL 
OUTPUT 



VOLTAGE 
FOLLOWERS 



Figure 8. A block schematic diagram of trequency 
shifter with built-in quadrature oscillator. 



quadrature oscillator provides a frequency shifting car- 
rier in the range from 0.5 to 5.0 Hz. The front panel 
layout of this feedback suppressor is shown in Figure 9. 

This instrument has also other interesting studio ap- 
plications. For instance it can be used as a pseudo stereo 
and ambience effect enhancement device, supplying a 
complementary signal for a second channel when fed 
with monophonic program material at its input. In Fig- 
ure 5, the frequency shifter is shown on top of the right 
hand equipment assembly. 

A FEW TYPICAL APPLICATIONS 

The typical applications of the model 735 can be put 
into four basic categories: 

1. The simple up- and down-detuning of sounds, in- 
cluding passes through zero shift and production of 
"mirror image." 

2. Frequency shift modulation around zero (or any 
other center frequency). 

3. In-step detuning with voltage controlled synthesizer 
modules. 

4. Repetitive detuning in tape loop. (Iteration effect). 
Here are some typical effects which can be obtained. 

Triggering an envelope follower in conjunction with an 
envelope generator from a drum sound source (which 
also connects to the signal input) and feeding the voltage 
contour obtained from the envelope generator to the con- 
trol input of the shifter will result in a varying frequency 
shift contour at the individual drum tone bursts (at a 
speed depending upon the decay time set at the envelope 
generator) and will yield a whole new class of sounds. 

By setting the main tuning control to zero and apply- 
ing a subsonic square wave to the control voltage input 
(linear mode), the up- and down-detuned outputs will 
switch places, resulting in a new type of special effect 
when heard over two channels. When this square-wave 
frequency is raised and enters the audio range, a com- 
pletely new effect is obtained. In addition, a number of 
other effects will be produced with different types of 





Figure 9. A front panel layout of anti-feedback fre- 
quency shifter. 



wave shapes applied to the control input. A sine wave in 
the order of 5-6 Hz will result in a stereo vibrato. A saw- 
tooth wave around 1 to 2 Hz will produce a somewhat 
dramatic effect. With pink noise applied to the control 
input,, the program material will assume a hoarse quality 
which can be remixed with the original program signal. 

By selecting the exponential mode and feeding the con- 
trol voltage of the keyboard controller of a synthesizer 
into the control input of the frequency shifter, an infinite 
variety of new harmonic and non-harmonic sounds can 
be obtained when feeding the synthesizer tone signal into 
the signal input of the shifter. In this mode, the shifter 
becomes an integral part of the synthesizer, capable of 
being programmed into a large number of systems con- 
figurations. 

A further special category of sounds obtained with the 
frequency shifter is the iteration effect, also referred to 
as the spiraling echo effect, which is produced by insert- 
ing the shifter in the line between the output of a re- 
corder to its input. In this setup, the delayed sound 
received at the playback head is frequency shifted, then 
rerecorded, played back and frequency shifted again 
and again. An increasingly detuned sound is created, the 
character of which is determined by the amount of tape 
delay and the amount and sign of detuning. Evidently 
other delay devices can be used, such as digital delay 
lines, acoustical delay lines and the like. 

The effects achieved with simple detuning of quasi- 
pitched sounds, such as drums, bells, and chimes cannot 
be overlooked; a frequency shifter can be a rather use- 
ful instrument with a drum section. Further applications 
Include the processing of the human voice and many 
other natural as well as synthesized sounds. 

The carrier injection model 750 can also be made into 
a rather versatile instrument by using a voltage-con- 
trolled oscillator (such as the Moog 921 ) for the carrier 
input. Almost all of the complex functions just de- 
scribed can be performed, with the exception of the fre- 
quency shifts through zero and modulation around zero 
shift. In the exponential mode, obtained through the 
Moog 921, which is controlled by the keyboard con- 
troller and which .supplies the carrier frequency, very 
rich sounds can be obtained when using outputs other 
than sine waves, such as the triangle, square wave or 
sawtooth waves. 

From these limited examples and from the preceding 
description it will become quite clear that a frequency 
shifter can be a most powerful tool for the production 
of new sounds. ■ 

REFERENCES 

1. Prestigiacomo, A. J. and MacLean, D. J. "A Frequency 
Shifter for Improving Acoustic Feedback Stability." Journal 
Audio Engineering Society, vol. 10 (1962), p. 111. 

2. Schroeder, M. R., "Improvement of Acoustic Feedback 
Stability in Public Address Systems." Proceeds of Third Int. 
Congress of Acoustics (1959), vol. 2, p. 771. 

3. Burkhard, M. D. "A Simplified Frequency Shifter for Im- 
proving Acoustic Feedback Stability." AES Journal, vol. 11, 
p. 234 (1963). 

4. Wayne, W. C, Jr., Baldwin Piano Company, Cincinnati, 
Ohio. U.S. Patent 3,004,460. 

5. Heck, L. and Biirck, Sudwestfunk, Baden-Baden, Ger- 
many, Patent 1,051,000. See also L. Heck, Gravesano Blatter; 
V. Ussachevsky, "Musical Timbre by Means of the Klangum- 
wandler," presented at the Sept. 1958 AES Convention. 

6. Bode, H. "Solid-State Audio Frequency Spectrum Shifter," 
presented at the annual AES Convention, October, 1965. 

7. Bode, H. and Moog, R. "A High-Accuracy Frequency 
Shifter for Professional Audio Applications." AES Journal, 
vol. 20 (1972), p. 453. 

8. Crowhurst, N. H. "Theory and Practice," db, The Sound 
Engineering Magazine, January, 1974, p. 10, 



www.americanradiohistorv.com 



NORMAN H. CROWHURST 



Feedback, part 3 



Interaction over two or more stages and special cases 
where amplification doesn't follow the rules are covered in 
the concluding study of feedback. 



THE BASIC riiEORY dcvcloped in the previous two 
parts is not too easy to apply directly. There is 
still a lot to cover. We could expand this to fill 
any number of parts; but instead we will try to 
give you the core of it all, in this third part. First, we'll 
recap the interaction concept that I developed back in 
1953, then we'll take a look at some of the things that 
theory overlooks. 

INTERACTION CONCEPT 

A single stage of amplification will always have one 
high frequency roU-otT and, if it uses capacitive or induc- 
tive coupling, it may also have a low frequency roll-off. 
Figure 1 shows a roll-off that occurs, in the absence of 
feedback, at 1000 Hz. If it occurs at 10 kHz or 100 kHz, 
or anywhere else, the effect is similar, just at a different 
place. 

Now, 6 dB of feedback pushes the 3 dB point up an 
octave. 12 dB of feedback will push it up two octaves, and 
so on. If you look at a low frequency roll-off the effect is 
similar: each 6 dB of feedback will push the turnover 
point down one octave. If it is direct-coupled, it goes 
down to zero frequency anyway. 

That is, in a sense, the starting point for the interaction 
concept. When you move to using feedback over two or 
more stages, the extension of frequency response is no 
longer so simple, but involves interaction between the 
stages, brought about due to phase effects. Figure 2 
shows the effect of two stages, each having a roll-off 
shown by curve (A). 

The two combined produce the response at curve (B), 
before any feedback. First, note that, as you add feed- 
back, while you extend the frequency response where it 
is level, just as you did with one stage (Figure 1), the 
ultimate roll-off does not change. Because you cut gain, 
you reach that ro 1-off later. This ultimate roll-off is repre- 
sented by line (D). 

Now the 6 dB octave, or half slope point, slides down 
a 6 dB /'octave line, (C), as feedback is changed. Here, 
with two identical roll-ofts at 2 kHz, each 6 dB of 
feedback extends the point of contact with the 6 dB/oc- 



NO FEEDBACK 














































































FEE 


3B« 


CI' 


c 














































































FEE 


5B/ 


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18 dB 


FEE 


OB 


no 


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24 dB 


FEE 


DB 


ac 


K 

































































































































































100 500 IK 5K lOK 40K 

Hz 



Figure 1. Effect of feedback when only one roll-off is 
effective. (From Crowhurst, N. H. Feedback, 1952). 



tave line by half an octave, which means it drops 3 dB in 
absolute level or, referred to the gain level with feedback, 
it rises 3 dB. 

Without feedback, this point is 6 dB down, at 2 kHz. 
With 6 dB feedback, it is 9 dB down at 2.828 kHz or, 
relative to the new level with feedback (curve E) 3 dB 
down at that point. This point is of interest in active filter 
design. Another 6 dB of feedback makes the touch point 
12 dB down at 4 kHz or, relative to the level with feed- 
back, it is zero dB at 4 kHz, with a peak of about 1.25 dB 
at 2.828 kHz. 

Going to 18 dB of feedback pushes the touch point up 
another half octave, but now it is up 3 dB from the level 
with feedback, with a peak of about 3.6 dB a little below 
that frequency. The last one shown, with 24 dB feedback. 



www.americanradiohistorv.com 




40K 



o 

UJ 

Li- 

O 
UJ 



,7 
.8 
.9 



25 - 



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IS 

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10 — 



5 — 



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80 - 
70- 
60- 
50- 

40^ 



20—: 



Figure 2. Effect of feedback with two Identical roll-ofts 
at the high end. (From Crowhurst, N. H. Feedback, 1952). 



Figure 3. An abac for calculating response details in 
feedback over two roll-offs. (From Crowhurst, N.H., 
High Fidelity Sound Engineering, 1961). 



makes the touch point up 6 dB, from level with feedback, 
at four times the original frequency, or 8 kHz, and about 
6.3 dB peak just below that. 

At the low frequency end, this pattern is exactly re- 
versed when you have two elements contributing to a low 
frequency roll-off within a feedback loop. Figure 18 is 
an abac to facilitate calculating what feedback does in all 
such cases. In Figure 2, we assumed identical roll-offs. 
That condition is a special one, which will not often ap- 
ply. Figure 4 shows how to apply Figure 3 in locating 
the response. 

You have two turnovers, spaced at a frequency ratio 
of n^. Midway between them, the response without feed- 
back will have a slope of 6 dB/octave. Applying feed- 
back will extend the touch point on the 6 dB/octave (or 
unit slope) by the extension factor, read across the left 
hand line in Figure 3. The chart can be used to find all 
reference points on the curve, given the necessary data. 

MORE THAN TWO STAGES 

Where feedback covers more than two stages, which is 
becoming more rare in these days of solid state circuitry, 
complete analysis becomes more complex, because the 
roll-offs can come in all kinds of combinations, not just a 
simple ratio. But we can simplify this by taking a best 
case, which under other circumstances can become a 
worst case. 

If you have three roll-ofts within a limited range of roll- 
to off points, say a 10:1 frequency range, the best case, for 
oJ achieving the most feedback without peaking first, and be- 

coming unstable second, is when one roll-off acts first, say 
o at 10 kHz with the other two acting at the other extreme, 
(5 in this case 100 kHz. With that combination (Figure 5), 
^ 1 1 dB will reach the peaking boundary (i.e. more than 1 1 
^ dB will cause a peak in the response) and 28 dB will make 

the circuit go into oscillation. 

From another viewpoint, that is a worst case. If the first 

two rolloffs to act are on a 10:1 ratio, the worst case is 
CO when the third one is also at the second of these fre- 



quencies. Putting the third one beyond the second fre- 
quency will improve the figures slightly, but not much, 
unless the third frequency is removed very much further 
out. 

Figure 5 tells the limits, but it does not tell what hap- 
pens in between, for which Figure 6 is helpful. To use 
this, you take the peaking boundary given by Figure 5 
as the amount of feedback allowable without peaking. dB 
Excess Feedback on Figure 6 is the amount of feedback 
more than this. Thus if n is 10, as in the example of the 
previous paragraph, excess feedback starts at 1 1 dB, and 
20 dB feedback would be 9 dB excess feedback, resulting 
in between 5.2 and 7 dB peak, probably about 6 dB. 

If the first roll-off is at 10 kHz with identical rolloffs, 
the peak would be at a little over 12 kHz, and with very 
large roll-offs, it would be at about 7 kHz. With the 10:1 
ratio it would be somewhere between these extremes. 

Earlier presentations extended these predictions to as 
many as five roll-offs within a feedback loop. This shows 
the method. If anyone wants me to go further with this, 
we can pursue it later. In the meantime, let us look at other 
aspects of feedback. 

WHAT THE THEORY DOESN'T SHOW 

All that theory is based on analyzing feedback per- 
formance, using frequency as a reference. It assumes that 
all components behave essentially the same throughout 
the waveform cycle. That is, amplification is essentially 
linear, and impedances due to dynamic active components 
do not change during the audio cycle. 

For class A amplification, that may be a reasonable 
assumption. But what about when an amplifying device 
runs into clipping^or cut-off? Then, although it is not 
behaving quite like a digital device, it does have two quite 
definite states during the audio cycle. For part of the cycle 
it is amplifying and for part of it, it isn't. 

What about class B amplification, where two transistors 
share the complete waveform, so that, when one isn't 
amplifying, the other one is? That may be okay for many 



www.americanradiohistorv.com 




purposes, provided the transistors match up, that is, pro- 
vided that each starts to amplify at precisely the same 
point on the wavcfonn as the other leaves ofT. Otherwise 
you have a iwo-slate system. 

Maihematical or theoretical analysis doesn't work too 
well anymore. Nor doe-, the digital form of two-state. Now 
you have lo treat the system qualitatively on a time-based 
analysis, considering what it tloes while the devices oper- 
ate at their normal amplifying level, and what it does 
•.luring those parts of the audio cycle where the ampli- 
licaiion quits. 

One example of this can occur in the relatively simple 
.single stage transistor amplifier, discrete. Look at Figure 
22. While it is amplifying, the bias current of the second 



Figure 5. Chart giving boundary conditions tor feedback 
over tijree stages of roll-offs. (From Crowtiurst, N. H., 
High Fidelity Sound Engineering, 7961^. 



stage holds it conducting, and the output side of the 
coupling capacitor is, say, a few hundred ohms from 
ground. Its input side couples from the collector of the 
preceding stage, whose maximum impedance is the value 
of the collector resistor, say 1 0 k". The following stage 
bias resistor is, say 560 k-. 

But because, while the transistor is conducting, that side 
of the capacitor sees a few hundred ohms to ground, the 



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dB EXCESS FEEDBACK 20 LOGio Fx 

Figure 6. Chart showing variation limits tor frequency 
and height of peak with circuits within boundary 
conditions of Figure 5. (From Crowhurst, N. H., 
High Fidelity Sound Engineering, 1961). 




Figure 7. A form of coupling that can give trouble 
not predicted by the theory. 



•lOK 



'lOK ^^50 



Figure 8. Another example: the well-known emitter 
follower. 



impedance associated with that coupling capacitor is about 
10 kSi. A 2 fiF capacitor will give a low roll-off of about 
8 Hz. But now the signal level rises enough to swing this 
stage so that the second transistor just reaches cut-off. Two 
things happen. 

All the while the transistor was conducting, the capaci- 
tor merely provided a.c. coupling. But as soon as it swings 
into its non-conducting region, it suddenly begins to act 
like a simple diode, using the 2 fiF capacitor to build up a 
charge that sends it further into non-conduction. 

And second, when it was conducting, that 8 Hz roll-off 
represented a time constant of 20 milliseconds. Now the 
relevant components are 2 and 560 k^-, which repre- 
sents a time constant of over a second. If you want the 
roll-off lower, as you probably would, the time constant 
is even longer. 

That particular problem is relatively simple to correct. 
Just put, say a 10 k*-' resistor from base to ground. When 
the amplifier is amplifying, it will have almost no effect, 
paralleling the base input resistance of a few hundred 
ohms. But when it cuts off, it limits the time constant to, 
say 40 milliseconds. 

That covers what happens at cut-off, in that instance. 
If you run into saturation, a similar thing happens; ampli- 
fication, relatively suddenly, disappears. The parameters 
that are operative while amplification is present and on 
which the formulas in the earlier parts of this series were 
based, no longer apply. 

What happens if you suddenly write zero for A, instead 
of whatever figure it has when amplification is operative? 
That cannot be expressed in a simple formula. You must 
look at the circuit and ask yourself what the condition is at 
each point around the circuit when this happens. You are 
referencing against time, not against frequency. 

MULTI-PURPOSE FEEDBACK 

For another example, take the well-known emitter fol- 
lower. Suppose (Figure 8) you have a previous stage 
that uses a 10 kf' collector resistor. You direct-couple this 
to an emitter follower with a beta of, say 40. By the sim- 



ple rule for emitter followers, this will produce a reflected 
impedance of 250^!. 

Now, that emitter follower can do two things — change 
impedance and reduce distortion. We have already used 
its impedance-changing property. Assume its emitter re- 
sistor is another 10 k^K With a beta of 40 and a 10 
k^'- emitter resistor, this puts its voltage amplification at 
40, and its feedback factor at 41. If, as an ordinary am- 
plifier stage, with no feedback, this transistor has a dis- 
tortion of 8 per cent, then as an emitter follower with a 
10 kS2 load, that ditsortion will be down to 0.2 per cent 
or slightly less. Since it is directly coupled, overall feed- 
back could be used to knock distortion even lower. 

But now, suppose you use the emitter follower so it can 
operate into the same source impedance it presents at the 
output, 250*2, What happens now? Now its emitter load 
is not 10 k!2, but 250^!. Its gain is down from 40 to 
around 1. So 1 -I- A/3 is 2, instead of 41. This means that 
the distortion figure is about 4 per cent, instead of 0.2 
per cent. Overall feedback may still knock it down, but it 
will be bigger, by about 20 times, than you expect. 

At frequencies where phase shift comes into the pic- 
ture, the distortion components, harmonic frequencies, will 
not be properly negative, so will not be reduced as much 
as calculated for other frequencies. 

Another form of multi-purpose feedback, used in audio, 
is where you use it for gain control. Now you are not 
using audio feedback, as such, but are processing it, to get 
d.c, that controls gain. Of course, your d.c. is filtered. But 
that filtering must meet certain time-constant demands, to 
meet specificatiojis. 

This means that it cannot perfectly eliminate the audio. 
You think of it as d.c. feedback, but there is some audio 
feedback present as well. Never forget that. 

This has been a sort of short course on feedback basics. 
I've tried to give you most essentials. How did you like it? 
Are there pieces you feel I missed? Let me know. ■ 



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each, postpaid. 3P Recording, P.O. Box 
99569, San Francisco, Ca. 94109. 



INFONICS DUPLICATORS! For a bunch 
of reasons, you can't afford not to con- 
sider Infonics Duplicators — especially 
since factory installation and training are 
included in the list price. INFONICS 
DUPLICATORS, (219) 879-3381. 



COMMUNITY LIGHT & SOUND profes- 
sional sound reinforcement products. 
Brondy Brook Audio, P.O. Box 165, 
Seymour, Conn., 06483. (203) 888- 
7702. 



otscooiffS'- 

Dhe^ & Audio Equiprrtetrt- 
Low Prices on Haw Equ^pmefit 
AinpQ,Twrjiiabl*Sp Miners A tap^- 
Dttcks by Thorens, DynMQ, 653^ 
Crown, T«cltnic«, GLI, MetAor, 
EECfTeac Amaniy mDr^ 

Rout* 3S, Eatontown IL JL 07724 
a01-54a-70CH> ' 



www.americanradiohistorv.com 



ORTOFON 
DYNAMIC MOTIONAL FEEDBACK mono 
disc cutting systems. Complete with 
drive, feedback, and feedback-playback 
monitor amplifiers and cuttertiead, All 
systems guaranteed. Spare cutterheads 
available for exchange/repair. Albert B. 
Grundy, 64 University Place, New York, 
N.Y. 10003. (212) 929-8364. 



DISCO MIXERS. Write for free bulletin. 
Berkshire Audio Products, P.O. Box 35, 
Great Neck, N.Y. 11021. 



FOR SALE: SPECTRA SONICS 1020 
console; 20 x 8 -f 4, 2 cue systems, on 
connectors ready to plug in, $12,500. 
Stantron racks, remote cables, power 
supplies, amps; best offer. Larrabee 
Sound Studios, 881 1 Santa Monica Blvd., 
Los Angeles, Ca. 90069. (213) 657-6750. 



NAB ALUMINUM FLANGES. We manu- 
facture 8", lOVi", & 14". For pricing, 
write or call Records Reserve Corp., 56 
Harvester Ave., Batavia, N.Y. 14020. 
(716) 343-2600. 



ORBAN/PARASOUND FACTORY CLOSE- 
OUT on used and discontinued Items. 
105C Spring Reverb, originally $595-$350 
used. 105A Reverb for ± 15 volts — $300 
used. 51 6E de-essers, originally $495- 
$300 new. Custom 4-channel octave- 
band graphic equalizer for tuning sys- 
tems (controls are trimmers on PC 
board)-$600 used. Also 2 Lang PEQ-2 
program equalizers, used, $250/ea or 
both for $450. Guarantee: 1 year on new 
equipment; 90 days on used. Terms: 
COD. All items subject to prior sale. 
Orban Associates Division, 459 Bryant 
St., San Francisco, CA 94107. (415) 
957-1063. 



PRO AUDIO EQUIPMENT « 
SERVICES 

Custom touring sound, 2-, 4- and 
8-track studios, disco systems. 
Representing Akai, AKG, Altec, 
Beyer, BGW, Cetec, Cerwin-Vega, 
Community Light & Sound, dbx, 
Dynaco, Dokorder, E-V, Gauss, 
Lamb. Langevin, 3M, Martex PM, 
Maxell, Meteor, Russound, Revox, 
Sennheiser, Shure, Sony, Sound- 
craftsman, Sound Workshop, Spec- 
tra Sonics, Switchcraft, TDK, 
TAPCO. TEAC, Technics, Thorens, 
and more. Offering these profes- 
sional services: custom cabinet 
design, room equalization, loud- 
speaker testing, custom cross- 
over design, electronics modifica- 
tion, and custom road cases. Call 
or write for quotes, or drop us a 
line for our latest catalogue. K & L 
Sound, 75 N. Beacon St., Water- 
town, Mass. 02172. (617) 787- 
4073. (Att: Ken Berger) 



FOR SALE 
Two MCI 416 consoles, 24-in/out. 
Input modules modified to include 
latest improvements for increased 
heardroom and lower noise dis- 
tortion. EQ modified for mid-range 
boost/cut function. Matching cus- 
tom producer's desk and connect- 
ing right angle rack for outboard 
gear included. Very impressive 
package. $19,000. each. 

Scully mono lathe. Westrex 2B 
mono head. Fairchild 660. Fair- 
child 600 hi-frequency Pultec EQP- 
1A. Pultec MEQ-5 mid-range. 
Haeco monitor and Cutter elec- 
tronics. Custom monitor panel. 
$5,000. 

Westrex 3D MA stereo cutter 
head and RA-1700/3D II AH am- 
plifier. Haeco low frequency cross- 
over and custom monitor panel. 
$7,500. 

CHEROKEE RECORDING STUDIOS 
751 N. Fairfax Ave. 
Los Angeles, Ca. 90046 



WE WILL BETTER anyone's price on 
new Recordex high speed cassette dup- 
licators. Your written request can save 
you a bundle. Also get our large-user 
cassette deal. Tape and Production 
Equipment Company, 2080 Peachtree 
Industrial Court, Atlanta, Ga. 30341. 
Phone (404) 458-TAPE. 



EQUIPMENT UPDATE; must sell: 4 Scully 
270s, 2 Gates carousels, 3 Gates cart 
decks and Gates SC-48 programmer. All 
equipment in excellent condition. All of- 
fers considered. Stan Gold, KJ01-FM, 
2555 Briarcrest, Beverly Hills, Ca. (213) 
278-5990. 



MCI . . . DOLBY. Two great names! 
Two great products! For authorized fac- 
tory representation in the progressive 
Midwest, contact: Jerry Milam, Milam 
Audio Co., 1504 N. 8th St., Pekin, III. 
61554. (309) 346-3161. 



WANTED 



WANTED: 3M SCULLY or Studer 2-track 
recorder. Top condition only. So. Calif, 
area only. Phone: (213) 461-3717. 



EMPLOYMENT 



OPPORTUNITY FOR AGGRESSIVE re- 
cording engineer and mixer. Must have 
a common sense business attitude and 
be a professional in every aspect of the 
business. We record and produce for 
America's largest labels. Outstanding 
facility: 16-track, dbx, etc. Applicant 
must be able to repair and perform 
maintenance. Send resume immediately. 
Box 31, db Magazine, 1120 Old Coun- 
try Rd., Plainview, N.Y. 11803. 



EXPERIENCED MUSIC MIXER 

For major N.Y.C. studio, expand- 
ing staff. Send resume to Box 11, 
db Magazine, 1120 Old Country 
Rd., Plainview, N.Y. 11803. 



WANTED: STUDIO MANAGER with 
proven ability to make money, to buy 
into and take over existing 16-track stu- 
dio in major East Coast city. Strong 
technical or musical background help- 
ful but not necessary. We can handle 
engineering, maintenance, musical and 
sales tasks, but need organization, lead- 
ership, a new console and other equip- 
ment. The studio has 16-track dbx, JBL 
monitors, Yamaha grand, 83, novel floor 
plan, and much potential. Make it your 
own. Box 32, db Magazine, 1120 Old 
Country Rd., Plainview, N.Y. 11803. 



AUDIO/FM ENGINEER desires part-time 
steady work in NYC. Design, installa- 
tion, maintenance. First phone, A.B. de- 
gree, own tools and test equipment. 
(212) 795-6616. 



WANTED, J.B.L. pro dealers to handle 
high end multi-channel consoles on ex- 
clusive basis. Designed to augment J.B.L. 
pro line of speakers and amplifiers. 
Theatre Sound, Inc. P.O. Box A.Q., New 
Haven, Conn. 06525. 



PRODUCT MANAGER 

We offer several positions of re- 
sponsibility in our organization for 
your product line marketing man- 
agement talent. Proven experience 
earns exceptional income with cor- 
porate benefits, promoting our 
lines to dealer and national rep 
organizations. These opportuni- 
ties require 50% travel, and pre- 
sent future growth potential in our 
growing operations. 

Background Music Products: 

Marketing background in sound 
contracting necessary. 
Audio/Broadcast Products: 

Exposure to broadcast field 
practices and marketing essential. 

Send resume in detail, giving sal- 
ary requirement and references. 

Roger Taylor, 
TELEX COMMUNICATIONS, INC. 

9600 Aldrich Avenue South 
Minneapolis, Minnesota 55420 

An Equal Opportunity Employer. 
Women will be considered equally 
with men. 



www.americanradiohistorv.com 



d^people/places/happeiiings 




STARLING WORTMAN OSTERGAARD 



• William L. Starling has been pro- 
moted to the post of western regional 
manager, professional products, for 
Capitol Magnetic Products, of Los 
Angeles. Mr. Starling who came to 
Capitol from Data Packaging Corp., 
was formerly field service manager 
for Capitol. 

• The newly created position of prod- 
uct manager, logging recorders at the 
Scully /Metrotech Division of the Dic- 
taphone Corporation has been filled 
by Leon A. Wortman. Mr. Wortman 
has spent more than 20 years in the 
audio industry, associated with the 
Ampex Corp. and operating his own 
consulting firm. 

• Wally Heider Recording of San 

Francisco has announced the appoint- 
ment of Gary Blohm as general man- 
ager. Mr. Blohm was formerly West 
Coast manager of administration and 
recording operations for Columbia 
Records in Los Angeles. Among his 
new plans for the San Francisco facil- 
ity are getting into radio drama pro- 
duction, more commercial recording, 
and service to students and commu- 
nity groups. 

• The establishment of an interna- 
tional products division has been an- 
nounced by the Hoppmann Corpora- 
tion. The new division will offer stan- 
dard components and accessory items 
to the communications market, port- 
able a/v displays, personnel training 
cassettes, and intercom systems. The 
new division is focalized by Horace 
Frenk and Ed Somerville. 

• Speedier delivery of Bang & Oluf- 
sen's audio products from Denmark 
to the U.S. is being effected through 
the use of jet air freight. Shipments 
which used to require three weeks to 
Chicago will now be delivered in 
one day. 



• Paul B. Ostergaard, of Caldwell, 
New Jersey, has been elected presi- 
dent of the National Council of 
Acoustical Consultants. The Council 
is a nonprofit association represent- 
ing acoustical consultants in the 
U.S. and several foreign countries. 
They are headquartered in Silver 
Springs, Maryland. 

• A new acoustical engineering con- 
sulting firm, DBH Acoustics, has 
been formed in Portland, Oregon by 
Lawrence G. Hopkins and Albert G. 
Duble, Jr. The address is 10211 S.W. 
Barbur Blvd., Suite 209, Portland, 
Oregon 97219. 

• Jack R. Smith has been elected to 
the position of Board Chairman at 
Globe Communications of Cleveland, 
Ohio. Mr. Smith was formerly a field 
engineer with the FCC. 




• Project personnel of the U.S. Fish 
and Wildlife Service are shown pre- 
paring to locate and record howls and 
other vocal response of wild wolves 
on a Uher 4000 portable open-reel 
recorder. A radio-collared wolf is 
tracked through the use of a re- 
ceiver which receives directional 
beeps. The point is to count the num- 
ber of wolves, etc. in remote areas. 



• Jack K. Daniel has been appointed 
director of marketing of the Vega 
Division of the Cetec Corporation. Mr. 
Daniel will have his headquarters in 
EI Monte, California. Before joining 
Vega, he was with Harris Communi- 
cations. 

• Warren & Hickey Sales Company 

of Redwood City, California has been 
appointed by University Sound as 
their representatives for northern Cal- 
ifornia and northern Nevada. Princi- 
pals in the sales firm are Don Warren 
and Bob Hickey. University Sound is 
a line of the Altec Corporation. 

• John Snell, formerly senior produc- 
tion engineer for the ABC Radio Net- 
work, has formed his own production 
and recording company. He will also 
serve as production/technical consult- 
ant to public relations firm DWJ As- 
sociates, Inc. of New York. Among 
Mr. Snell's assignments while at ABC 
were political conventions and elec- 
tions, as well as several Gemini and 
Apollo space shots. His firm is located 
at 295 Madison Ave., New York City. 

• New western representatives for 
Analog & Digital Systems, Inc. of Wil- 
mington, Mass. have been appointed. 
The Henry Joncas Company of Seat- 
tle, Washington represents the north- 
western region and the mountain 
states are now served by MF Sales of 
Arvada, Colorado. 

• Offering a line of studio accessories 
in addition to studio design and con- 
struction services, Windt Audio Engi- 
neering is now operating from a new 
facility. The new office is at 13026 
Saticoy St. (#4), N. Hollywood, Cali- 
fornia. John Windt is the owner. 

• Synapellas, a series of quick syn- 
thesizer/ a'capella jingles, are being of- 
fered by WAY Audio Creations of 
Buffalo, N.Y. The tapes feature Roger 
Luther playing the world's largest 
Moog synthesizer and are designed for 
any uptempo music format. Demo 
tapes are available from Way Crea- 
tions, P.O. Box 21, Station B, Buffalo. 
N.Y. 14207. 

• Of interest in applications requir- 
ing background music is the "Index 
Series" recently introduced by Musi- 
Cues of New York City, representing 
the Chappell Background Music Li- 
brary. The series includes thirty-six 12- 
inch LPs and a compact catalogue, or- 
ganized with one LP per subject mat- 
ter. MusiCues is at 1156 Avenue of 
the Americas, New York, N.Y. 10036. 



www.americanradiohistorv.com 



THE PEAVEY 
800 STEREO MIXER 

Compare the advantages! 



The Peavey 800 S is, without question, 
the best mixer buy on today's market. Compare 
its features with those of other mixers in its 
price range: 

Eight channels with the very latest variable 
negative feedback circuitry. 

Each channel features seperate low & high 
equalization; pre & post capability for monitor, 
reverb, and effects send controls; attenuation; 
stereo pan; and slide level control. 

Master section features slide level controls 
for left and right main & monitor; low, mid, 
and high equalization for left & right mains; 
master level, return, and pan controls for the 
effects and reverb busses; and two lighted VU 
mclers with screwdriver adjustment. 



Rear panel features eight low (600 ohm) 
inputs and eight high (50 K ohm) inputs; left 
and right main & monitor outputs; auxiliary 
input panel, and a stereo phone jack for taping 
out. 

Suggested retail price: $649.50 at your 
Peavey dealer. 



7^/ 



Peavey Electronics, Corp. / Box 2898 / Meridian, Mississippi 39301 
Circle 1 1 on Reader Service Card 
www.americanradiohistorv.com 



\bu make it professional. 



You provide the talent and 
our new half- inch 8-track will 
do the rest. You get full 
frequency response 
in the sync mode, 
and integral DBX interface 
is available optionally— 
8 tracks and then some. 

The 80-8 



Full IC logic circuitry including motion 
sensing gives you positive, smooth control 
over all transport functions. And with 
automatic sync switching, overdubbing and 
punching-in are easy. 





So is routine maintenance. Remove two 
front panel screws and the meter section 
swings down to give you immediate access 
to the EQ, bias, and level controls. 



Everything you need to produce a com- 
mercial product. At a price very much in 
keeping with the whole tascam idea: 

Less than 
$3000.00' 

So if you've been wanting to go 8-track, 
wishing there was a way. ..there is. Check 
out the 80-8 at your TEAC Tascam Series 
Dealer -just call (800) 447-4700 or in Illinois, 
(800) 322-4400, to find the one nearest you. 



•Nationally advertised value. Actual resale piices will be 
determined individually and at the sole disci"etion of authorized 
TEAC Tascam Series dealers. 



TEAC. 

TASCAM SERIES 

TEAC CoiTXiration of America 7733 Telep aph Road. Montebello, Ca. 90640 ©TEAC 



www.americanradiohistorv.com